development of a single photon detector with wavelength shifting and light guiding

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DEVELOPMENT OF A SINGLE PHOTON DETECTOR with wavelength shifting and light guiding technology Dustin Hebecker Masterarbeit in Physik angefertigt im Physikalischen Institut vorgelegt der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn September 2014

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Page 1: DEVELOPMENT OF A SINGLE PHOTON DETECTOR with wavelength shifting and light guiding

DEVELOPMENT OF A SINGLE PHOTONDETECTOR

with wavelength shiftingand

light guiding technology

Dustin Hebecker

Masterarbeit in Physikangefertigt im Physikalischen Institut

vorgelegt derMathematisch-Naturwissenschaftlichen Fakultät

derRheinischen Friedrich-Wilhelms-Universität

Bonn

September 2014

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1. Gutachter: Prof. Dr. Marek Kowalski2. Gutachter: Prof. Dr. Ian Brock

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Abstract

This thesis is aimed at the development of a new single photon detector unit for in-ice neutrino experi-ments. The primary development target is to reduce the noise of each detection unit while significantlyincreasing their efficiency in comparison to previous units like the digital optical module (DOM) at acomparatively low production cost. With these improvements the detection unit will allow to detect su-pernovae neutrinos from outside the milky way, while increasing the overall performance of the detector.The proposed module makes use of wavelength shifting and light guiding technology and is thereforecalled the wavelength shifting optical module (WOM). Main focus in this thesis is given to the devel-opment of a wavelength shifting paint with absorption between 250 and 400 nm and emission abovethat range. Several techniques to apply and evaluate the paints and the detector properties have beendeveloped. Suitable light guides have been investigated and an appropriate paint could be developed. Afirst approximation of the final module efficiency has been made and shows an increase in efficiency of170 % compared to the DOM efficiency. However, the detection units efficiency has not yet reached thetheoretically possible optimum and further studies are necessary.

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Contents

1 Introduction 1

2 Theoretical Background 32.1 Neutrino Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Detection Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 The IceCube Neutrino Telescope . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.3 Digital Optical Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 A New Detection Unit - The Wavelength Shifting Optical Module . . . . . . . . . . . 102.2.1 Liouville’s Theorem (Phase Space Conservation) . . . . . . . . . . . . . . . . 112.2.2 Wavelength Shifting Paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3 Snell’s Law, Fresnel Equations and Implications for the WOM . . . . . . . . . 132.2.4 WOM Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Laboratory Setups 173.1 Dip Coater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2 Absorption Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.1 Light Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.2.2 Sample Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3 Determination of Emission Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.4 Efficiency Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.5 Position Depending Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.6 WLS Quantum Efficiency Measurement Setup . . . . . . . . . . . . . . . . . . . . . . 293.7 Camera Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4 Commercial WLS Paint 334.1 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.1.1 Paint Layer Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.1.2 Absorption Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.1.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5 Development of Wavelength Shifting Wave Guiding Tube 415.1 Development of Wavelength Shifting Paint . . . . . . . . . . . . . . . . . . . . . . . . 41

5.1.1 First Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.1.2 Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

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5.1.3 Binder (Plastic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.1.4 Wavelength Shifter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.2 Paint Substrates - Wave Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.3 Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6 Full Module Efficiency 59

7 Conclusion 637.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

A Useful information 65A.1 Dye theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65A.2 Additional Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Bibliography 71

List of Figures 75

List of Tables 81

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CHAPTER 1

Introduction

Since pre-historic times, humans have been gazing at stars and wondered about their nature. When thefirst ancient civilizations emerged, astronomy became essential in their social, political, and religioussystems. It is even considered to be the first science ever, although at that time usually entangled intheological believe systems. Prominent examples for such civilizations and the earliest confirmed datesfor conducting astronomy are: the ancient Egyptian (4000 B.C.E.), Mesopotamia (3000 B.C.E. andpossibly earlier), Chinese (3000 B.C.E.), Britain (3000 B.C.E.), Indian (2000 B.C.E.) and Mayan (275B.C.E.) [1].Since then humans kept improving methods and technologies for star gazing. The most important in-ventions are the telescope in 1608 C.E. [2] and radio astronomy in 1930 C.E. [3]. However, these kindof observations focus only on the electromagnetic spectrum. It is desirable to make use of differentmessenger particles, because they have different advantages and disadvantages, and combining mul-tiple messengers yields an increased amount of information. Cosmic rays consist also of non-photonicparticles that are mostly charged. Due to the presence of non-quantified magnetic fields in the Universetheir paths are changed and the information about their origin can not be retrieved. It is therefore notstraight forward to determine the sources of high energy cosmic rays. Neutrinos are neutral particles andalso part of the cosmic ray spectrum. Like photons, they are not deflected by magnetic fields, and unlikephotons they are practically not absorbed by obstacles on their path. They allow for the identification ofcosmic ray sources. On the contrary, they interact very seldom. Therefore, huge detection volumes arenecessary to build neutrino telescopes. To afford volumes of this size, natural water and ice reservoirsare used as target material. After primary interaction, secondary charged particles are produced that canbe detected via Cherenkov light in the water or ice (section 2.1.1), using detection units deployed withinthe detection medium.These neutrino observatories also allow for indirect dark matter searches, observation of atmosphericneutrinos, general cosmic ray observations and related science goals. However, concerning supernovaedetection via neutrinos, current detectors are only sensitive to supernovae in or very close to our owngalaxy. In order to observe supernovae that are further away, the energy threshold and the noise rate ofcurrent neutrino detectors needs to be significantly decreased1. Via simulation of a large volume lowenergy in-ice detector at the IceCube site [4] the noise threshold for this kind of detection is estimated tobe 10 Hz per detection unit. The detection units employed currently by IceCube are based exclusively

1 This concerns detectors of ≈ 1 Mton volume. Some smaller detectors have the necessary resolution, but do not have thenecessary detection volume, to detect enough neutrinos.

1

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1 Introduction

on photo multiplier tubes (PMT). Their noise rate is about 500 Hz to 800 Hz and is directly proportionalto the PMTs photosensitive area. Also the amount of detected photons per detection unit should beincreased to lower the energy threshold and improve the energy resolution. This stands in contrast tothe need of lower noise rates and the proportionality of the noise rate to the detection area of PMTs.When a detector with higher effective area and lower noise rate can be built, this would not only allowfor better supernova detection but also a faster determination of the neutrino mass hierarchy comparedto current efforts with the planned PINGU extension of IceCube [5]. Therefore it has been looked at thelimitations of existing detection units to learn how to overcome them. By combining old techniques innew ways, a innovative detector design has been created. This thesis is about exploring and developingtechniques to actually implement this idea. The first challenge for the detector concept has been theenlargement of the effective area while improving the signal to noise ratio. In order to accomplish this,instead of using a PMT as primary detection area a light guide is used. As a direct consequence ofLiouville’s theorem (section 2.2.1) the product of solid angle and cross-section of the incoming lightis conserved, thus the detection area is not increased. To overcome this limitation the light guide iscovered with wavelength shifting paint (section 2.2.2), to couple the light into the light guide. To detectthe wavelength shifted photons at both ends of the light guide, small PMTs with considerably lowernoise can be used. The amount of photons in Cherenkov light (section 2.1.1) is notably higher in theultraviolet (UV) part of the electromagnetic spectrum. Therefore the detection range of the detectorshould go to the lowest wavelength possible. Limiting that is the transparency of the surrounding pres-sure vessel and the detection medium. The pressure vessel could be made of quartz glass to be mosttransparent to UV light while still strong enough to withstand the pressures of 3 km antarctic ice2.Quartz glass sets the lower limit to the necessary absorption range of the wavelength shifting paint toabout 250 nm. The upper limit of the length shifting paints shifting range is limited by the acceptanceof the used PMTs. The PMTs are most efficient towards the blue spectral range. Therefore the lightshould be shifted to wavelengths just above 400 nm and the absorption range should end below that tominimize re-absorption.

At the beginning of this thesis a short introduction to neutrino detection with a focus on the IceCubedetector is given (section 2.1) to familiarize the reader with the concept. Following that the new detectordesign and necessary components are introduced (section 2.2.1). The main part of this thesis focuseson the basics of paint application and efficiency determination (chapter 3), followed by first attemptswith a commercial wavelength shifting paint and the development stages of a wavelength shifting paint(chapter 5) matching precisely the requirements of the above described detector. Different materials forthe light guide will be investigated (section 5.2). Obstacles encountered during the development if notsolved yet will be discussed shortly (section 5.3) and possible approaches suggested. In the end, basedon the results from the wavelength shifting paint the expected efficiencies of the detector are calculated(chapter 6) and an outlook on the next development stages is given (chapter 7).

2 Due to UV-absorption, water detectors can not be built based on this concept.

2

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CHAPTER 2

Theoretical Background

2.1 Neutrino Telescopes

Figure 2.1: Neutrino cross section across energy scales [6]. As concrete example only the electroweak cross-section νee− → νee− is shown. The peak at 1016 eV is caused by the Glashow resonance [7].

Neutrinos have a very low cross-section1, like shown in Fig. 2.1. While the cross-section rises withthe energy of the neutrino the flux2 of astronomical neutrinos decreases [8], [9]. Therefore it is desir-able for neutrino detectors to have a large detection volume. For detection with Cherenkov radiation(section 2.1.1) the detection material must also be optically transparent. The cheapest, abundant and op-tically transparent materials are water and ice. Therefore all neutrino telescopes with at least a megaton

1 Cross-section is the likelihood of an interaction event between two particles and inverse to the interaction length Fig. A.2.2 The flux in this context is the amount of particles passing per unit of time and steradian through a defined area.

3

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2 Theoretical Background

detection volume use either of these materials. The first detector of this kind was DUMAND (Deep Un-derwater Muon And Neutrino Detector Project [10]) off the shore the island of Hawaii, operating from1976 to 1995. Since then many other similar experiments have been set up. Among others, the lakeBaikal deep underwater neutrino telescope in Russia [11], Amanda (the predecessor of IceCube) at theSouth Pole operated until 2008 [12], Antares [13] (the predecessor of KM3NET) in the MediterraneanSea, IceCube [14] at the Amundsen-Scott South Pole station and KM3NET [15] planned at three sitesin the Mediterranean Sea. All these experiments aim to detect high energy (> 100 Gev [16]) neutrinosand possibly their sources. But the scientific goals are manifold and go far beyond pure astronomicalinterests. Depending on their sensitivity and resolution also questions of modern particle physics areaddressed. They aim to determine the neutrino mass hierarchy [17] and search for dark matter [18].Even neutrino tomography of the earth for the determination of its internal structure is conducted. In thenext sections the fundamental physical processes used for neutrino detection (section 2.1.1), IceCube asexample for neutrino detectors (section 2.1.2) and IceCubes detection units, the digital optical modules(DOM) (section 2.1.3), will be presented. Afterwards the concept of an alternative detection unit, thewavelength shifting optical module (WOM), will be presented and motivated (section 2.2).

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2.1 Neutrino Telescopes

2.1.1 Detection Process

This section describes the neutrino detection process in water and ice based neutrino telescopes, bygiving a short overview on the participating processes.

Neutrino Interaction

Neutrinos have a mass of∑ν mν < 1.8 eV [19] with ν denoting the neutrino flavour. They are electrically

neutral leptons which only interact weakly. Therefore they can not be observed directly. When theyinteract they either interact with hadrons or leptons. the corresponding processes are depicted in Fig.2.2. In the hadronic CC case a high energy lepton and a hadronic shower is produced and in the NCcase only a hadronic shower. The hadronic showers are always produced by the high energy quark. Inthe leptonic CC and NC case a high energetic electron is produced. The leptonic interactions have alower cross section than the hadronic ones, except for the energy band between 3 and 10 PeV (Glashowresonance [7]). All produced charged particles then produce Cherenkov light, as long as they meet thenecessary energy requirements, like described in the following section.

(−)ν l

q′

q (−)ν l

q

(−)ν l

Z0

q′

νe e−

e−

W+

νe

νe e−

νe

Z0

e−

Figure 2.2: Feynman diagrams showing the reaction of a neutrino with the particles in the ice. Charged current(CC) interactions are shown on the left and neutral current (NC) interaction on the right.The top row holds thegraph of hadronic interactions of neutrinos with a quark of a nucleus. On the bottom interactions of neutrinoswith shell electrons are depicted. Time goes from bottom to top.

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2 Theoretical Background

Cherenkov Radiation

When a charged particle moves through a dielectric medium the electric field of the particle changes theorbit shapes and therefore the charge distribution of the atoms and molecules around it. As long as theparticle moves slower than the speed of light in that medium the energy loss of the particle by this effectis compensated by the energy gain from the relaxation of the polarized material while the particle movesaway. If the particle is faster than the speed of light (in particular the phase velocity) in the medium, theenergy from the relaxation of the depolarizing medium can not reach the particle anymore due to theelectromagnetic nature and speed limitation of the relaxation. This energy is then emitted coherentlyas so called “Cherenkov light”. Cherenkov light is emitted in a cone, where the opening angle can bedetermined as shown in Fig. 2.3. The red arrow represents the direction of the moving particle withvelocity βc. The green arrows represent the shock front of the emitted Cherenkov light. The speed oflight in the medium is c

n , where n is the refraction index of the medium. Then the opening angle isgeometrically defined as:

θc = arccos1

nβ(2.1)

Furthermore the photon number per unit length per wavelength behaves as described by the Frank-Tammformula [20]:

dN2

dxdλ=

4πz2e2

hcλ2

(1 −

1n2(λ)β2

)(2.2)

=2πz2α

λ2 sin2 θc. (2.3)

In eq. (2.3) α is the fine structure constant, λ the wavelength and z the number of elementary chargese of the moving particle. Integrated over a particular distance l and wavelength range λ1 − λ2 the totalnumber N of Cherenkov photons is:

N = 2 · παl(

1λ2−

1λ1

)sin2 θc, (2.4)

Figure 2.3: Diagram of a Cherenkov light cone. In red, the direction of the particle traveling with velocity βc > cn

the phase velocity of light in the traversed medium. In green, the resulting Cherenkov light wave front is shown.

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2.1 Neutrino Telescopes

The produced Cherenkov light can than be detected in neutrino telescopes like IceCube, which isdescribed in the following section.

2.1.2 The IceCube Neutrino Telescope

Figure 2.4: Sketch of the IceCube detector setup [21]. The main IceCube detector including the DeepCore sub-array consists of 86 sensor strings with 60 sensors each. On the surface is the air shower/veto detector IceTopconsisting of 81 stations with two ice tanks and two PMTs (Photo Multiplier Tubes) each. The IceCube prede-cessor, AMANDA II is shown within the IceCube detector volume.

The IceCube detector consists of roughly one cubic kilometer out of arctic glacial ice which is instru-mented with 5160 sensor modules. Fig. 2.4 shows a schematic sketch of this setup. The detector is splitinto three sub-detectors: IceCube, the main detector, IceTop the cosmic ray detector on the ice surface,that can also be used as veto for IceCube and DeepCore, a region with higher sensor density in thecenter of the detector that has a lower energy threshold. The 5160 DOM sensor modules, described inmore detail in section 2.1.3, are distributed among 78 IceCube and 8 DeepCore strings with 60 sensorseach. The horizontal distance between two ordinary strings is 125 m and the vertical distance betweenthe modules on the strings is 17 m. The modules were deployed in a depth between 1450 m and 2450 musing a hot water drill [14]. In this depth the pressure is so high that there are no cavities of air enclosedin the ice, because high pressure makes it energetically more favourable to enclose it within the crystalstructure of the ice. This makes it optically very transparent (apart from a dust layer at about 2050 m[22]), with an absolute absorption length ranging from 35 m to 230 m and a scattering length between4 m and 70 m [23]. A event recorded with IceCube is shown in Fig. 2.5.

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2 Theoretical Background

Figure 2.5: An event observed in August 2011 [24], the so called “Bert”-event named after the character in SesameStreet. Each sphere represents a DOM in the detector. The colors represent the arrival times of the photons withred indicating early and blue late times. The size of the spheres is a measure for the recorded number of photo-electrons. The reconstructed energy is 1.04 ± 0.16 PeV. [25]

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2.1 Neutrino Telescopes

2.1.3 Digital Optical Module

Figure 2.6: Schematic of an IceCube DOM (Digital Optical Module) [21]. Showing the glass housing surroundingin the lower part a PMT shielded by the mu-metal grid embedded in RTV gel and the electronics in the upper part.The electronics splits up in the HV divider at the top, LED flasher board in the upper center and the dalay aswell as the main board in the center. On the upper half the penetrator is fixed to allow for power input and signaltransmission.

The Digital Optical Modules (DOMs), are the detection units in IceCube. A schematic is shown inFig. 2.6. DOMs consist of a large pressure vessel, 33 cm in diameter and made of borosilicate glasswhich is low in potassium to reduce background radioactivity from 40K β-decays. The hull is 1.25 cmthick to resist the enormous pressure of up to 2450 m water depth and the additional forces applied bythe freezing ice. The photon detection happens in the lower part of the sphere, which is occupied bya hemispherical photo multiplier tube (PMT) of 25 cm diameter that has a noise rate of about 500 Hz[26]. Here, the Cherenkov light produced by secondary particles from the reaction of neutrinos withthe ice is detected. The PMT’s detection surface is facing downwards to have higher sensitivity forneutrinos coming through the earth. A layer of room vulcanizing (RTV) gel between hull and PMTprovides optical coupling and ensures low reflectance of incoming photons (section 2.2.3). A mu-metalgrid shields the PMT from earths magnetic field. The upper half of the sphere is filled by electronics.Through a penetrator the DOM is connected via cable to the IceCube laboratory. The electronics consistof a high-voltage supply (HV) for the PMT, six light emitting diodes (LEDs) of 405 nm for calibrationand the readout electronics. The latter in turn contain a control and a trigger system. Also included are adigitization unit (to reduce data transfer rates and therefore cross talk on the cable) and a delay unit. Allthe logic and calculation processes are implemented on a field-programmable gate array (FPGA) whichincludes a small ARM9-type computation unit (CPU).

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2 Theoretical Background

2.2 A New Detection Unit - The Wavelength Shifting Optical Module

To be able to detect supernovae beyond our own galaxy with neutrinos in IceCube, it is necessary toincrease the amount of detected photons per detection unit while decreasing the noise rate[4]. This willdecrease the lower energy detection threshold and increase the energy resolution. From that not just thesensitivity for supernovae is increased but the overall sensitivity, thus all physics goals profit. Thereforeit is desired to construct a detection unit with this capabilities for further IceCube extensions.With the current detector (DOM) concept these properties can not be archived. The size of the DOMcan not just be increased to achieve higher photon counts, because this would increase the productioncosts dramatically. Also for PMTs the noise rate is proportional to its photosensitive area thus the signalto noise rate is not increased with a bigger DOM.Therefore a new concept has been developed which can be seen in Fig. 2.7, it is called the wavelengthshifting optical module (WOM). It uses two classical PMTs which have a smaller detection area andthus produce lower noise rates. The photosensitive area is increased by using a wavelength shiftinglight guide as additional passive component. The pressure vessel is made of quartz glass with a lower40K content than borosilicate, thus reducing the noise level further. Quartz glass is also transparent toUV-light up to ≈ 250 nm, which is shown in Fig. A.3. This enhances the amount of detectable photons,as it was previously shown (section 2.1.1) that the Cherenkov spectrum is proportional to ∝ 1

λ2 .In the next section it will be explained why the wavelength shifter (WLS) is essential to the capture ofmore photons.

Figure 2.7: Schematic of the Wavelength shifting Optical Module (WOM) on the right [27]. The working principleof a wavelength shifting wave guide is shown on the left. Light is absorbed by a thin layer of wavelength shifter(WLS) and re-emitted isotropically at a lower wavelength. With a certain probability the re-emitted light iscaptured by total internal reflection and guided towards the PMT.

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2.2 A New Detection Unit - The Wavelength Shifting Optical Module

2.2.1 Liouville’s Theorem (Phase Space Conservation)

Liouville’s theorem states that the phase space distribution function of a dynamical ensemble is constantalong all temporal trajectories.3 Furthermore it states that the volume of any subset in phase space isconstant in time and even conserving orientation. Mathematical proofs can be found in [28].

In order to couple light into the light guide (sideways) it is necessary to use a WLS. As Liouville’stheorem states, phase space is always conserved. This means that without a WLS, the product of capturearea and capture angles is conserved. Thus the isotropic light could not be focused onto a small PMT.By using WLS the light gets absorbed and is isotropically re-emitted at a different wavelength withinthe WLS paint. This way the phase space states of the light changes not just in its angular and areaproperties but have to be regarded as a combination of the WLS molecule and light phase space. Thisallows to capture the light with larger area and focus the shifted light to a smaller area, while conservingthe overall phase space.

2.2.2 Wavelength Shifting Paint

Previous tests showed that if the WLS is contained in the whole light guide, re-absorption of the emittedlight is to strong. Also evaporating WLS onto the tube caused the WLS to crystallize and decreased thewavelength shifting efficiency. Therefore it was chosen to apply the WLS in a wet film by a dip coatingprocess. The basics of paints and WLS will be presented in the following two sections.

Paint

Paint is always made of at least two materials, a solvent and a binder. In most cases also pigmentsfor color are added. In relation to this thesis, wavelength shifters (WLSs) are added as pigment. Thesolvent is usually a liquid that solves a solid, here the binder and sometimes the pigment as well. Asolvent interacts with the binder at a molecular level and breaks out molecules from the binder solid tocreate a solution.Most solvents have another property that is useful to create a paint, they evaporate quickly. Hence, ifthe solution is applied to a substrate4, the solvent evaporates and leaves an evenly distributed film of thesolid binder material at best. There are two ways to add pigments to the paint. One is to have very smallcolor pigments that are as dense as the solution, such that they distribute evenly in it and are later alsoevenly distributed in and on the paint. This is not possible for WLS as most of them tend to crystallizeand reducing their ability for wavelength shifting. The second way is for the pigments to also be solvedby the solvent like the binder. Therefore, if the amount of binder is much higher than the amount ofpigment, after evaporation of the solvent the molecules of the pigments will be enclosed by the binderand the pigments have no means to crystallize.

Wavelength Shifter (WLS)

A scintillator is a material that emits photons following excitation or ionization by radiation. WLSs areare therefore a special kind of scintillator, they are excited by light at short wavelengths and emits thenlight at longer wavelengths. The residual energy is transferred to the surrounding medium as lattice

3 The phase space is a multi-dimensional space created by all parameters of a system necessary to describe it completely.4 The base material the paint is applied on

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2 Theoretical Background

vibrations, so called phonons. The most common5 form of WLS are Fluorescence materials. The timebetween absorption and (re-)emission is in the order of a few ns.

Figure 2.8: Visualization of the state changes necessary for wavelength shifting, explaining the difference inabsorption and emission wavelength. [29]

The reason for absorption and emission at different wavelengths can be explained with the Franck-Condon principle. A visualization of this is shown in Fig. 2.8. The absorption energy does not justexcite an electron from one energy level to another but also to a higher state. The electrons are boundin a potential that can be described by a harmonic oscillator in a semi-classical model. They exhibitdistinct energy levels. If they have a certain overlap in position, as described by the Franck-Condoncoefficient S 2

ν′ν′′ = 〈ν′|P|ν′′〉 , 0, transition is possible. When ν′ , ν′′ wavelength shifting can beachieved. This is possible since the excitation happens on much shorter time scales (O(10−14s)) thanthe change of nuclear coordinates (O(10−12s)). They in turn decay comparatively faster than the excitedsinglet state (O(10−8s)) [30], causing them to decays earlier. When the singlet state decays the energeticpotential is already much smaller and the emitted wavelength for the de-excitation is longer.A system has usually many discrete absorption and corresponding emission lines. In liquids and solidsthey are usually in-homogeneously broadened and create a single homogeneous spectrum. The differ-ence of wavelength between the maximum of the absorption and the maximum of the emission is the socalled Stokes-Shift as shown in Fig. 2.9. For WLS a high Stokes-Shift is desired. When the Stokes-Shiftis not large enough the overlap between emission and absorption spectrum increases, as shown in thegreen part of the figure and the medium becomes in-transparent to its own light. That in turn leads to alower conversion efficiency for the photons. More about the wavelength associated with a WLS can befound in section A.1.

5 Phosphorescence materials and non-linear crystals are also WLS but are not of interest in this context.

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2.2 A New Detection Unit - The Wavelength Shifting Optical Module

Figure 2.9: Illustration of Stokes-Shift. In blue the Absorption spectrum, yellow the emission spectrum and greenthe overlap of both which leads to re-absorption, ergo in-transparency and losses in efficiency for a WLS.

2.2.3 Snell’s Law, Fresnel Equations and Implications for the WOM

In order to understand the light losses while light changes from one medium to another and the capturingof light in the light guiding tube of the detector, Snell’s law and the Fresnel equations are essential.

Snell’s Law

Like Snell’s law states in eq. (2.5), when a light beam enters a medium of higher refraction index it isdeflected towards the normal of the surface and vice versa. This becomes impossible when the light isin a medium of high refraction index with an angle almost parallel to the surface. In those cases thelight is always reflected at the surface and captured within the material of high refractive index unlessits angle towards the surface changes dramatically. The angle where this starts to happen is called thecritical angle and only depends on the refractive index of the two media. The range for total internalreflection (TIR) is of special interest for light guiding. The critical angle under which TIR occurs can bedetermined from eq. (2.5) by setting sin θt = 1, as shown in eq. (2.6). This combination of steep angleand difference in refractive index can be used to transport light with low losses like it is done in opticalfibers for (tele-)communication purposes.

Snell’s law:sin θi

sin θt=

n2

n1(2.5)

Critical angle of refraction:θc = arcsin

n1

n2(2.6)

Fresnel Equations

The Fresnel equations as stated in in eq. (2.7) and (2.8) are essential for the determination of lossesfor light reflection and the traversing of optical media. They describe the probability of an incomingphoton to be reflected on a surface depending on the refraction index of the two materials involved andthe angle of incidence.

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2 Theoretical Background

Fresnel equation for s polarization:

Rs =

∣∣∣∣∣n1 cos θi − n2 cos θt

n1 cos θi + n2 cos θt

∣∣∣∣∣2 (2.5)=

∣∣∣∣∣∣∣∣∣n1 cos θi − n2

√1 −

(n1n2

sin θi)2

n1 cos θi + n2

√1 −

(n1n2

sin θi)2

∣∣∣∣∣∣∣∣∣2

(2.7)

Fresnel equation for p polarization:

Rp =

∣∣∣∣∣n1 cos θt − n2 cos θi

n1 cos θt + n2 cos θi

∣∣∣∣∣2 (2.5)=

∣∣∣∣∣∣∣∣∣n1

√1 −

(n1n2

sin θi)2− n2 cos θi

n1

√1 −

(n1n2

sin θi)2

+ n2 cos θi

∣∣∣∣∣∣∣∣∣2

(2.8)

As the in this experiment polarization is not of interest the Fresnel equations can be written like:

R =Rs + Rp

2(2.9)

for randomly polarized light. In the equations (2.5) to (2.8), n1 and n2 are the refractive index of the firstand second medium respectively, θi the incident angle and θt the out going or refracted angle. Equation(2.9) has been plotted in Fig. 2.10 and inscribed for better understanding.

Figure 2.10: Visualisation of the Fresnel equations (transmittance/reflection and their angular dependence). Onthe left side the reflection coefficient for randomly polarized light coming from a refraction index n = 1 and goingto a refraction index n = 1.5. On the right side the same for light going from n = 1.5 to n = 1. Important is thetotal internal refraction range. All the light within that angular range is captured inside the medium unless theangle of incidence changes.

Capture Efficiency of a WLS Coated Tube

From this the capture efficiency for the WOM can be determined. A tube like shape has been chosen forthe light guide. The capture efficiency for tubes is expected to be higher than the one for a combinationof bars or similar shapes, because sharp edges are avoided. The actual capture efficiency has not beencalculated yet. However, it is possible to determine a estimate on the capture efficiency. Therefore thetwo extreme situations in terms of the wall thickness are considered. For a wall thickness t → 0 the tube

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2.2 A New Detection Unit - The Wavelength Shifting Optical Module

can be locally approximated as a flat surface and the capture efficiency of infinitely extended glass platecan be used as illustrated in Fig. 2.11. This can be calculated from the critical angle as seen in eq. (2.6)and the solid angle from eq. (2.10) with ω the opening angle on one dimension. When accounting forboth sides of the plate the efficiency can be calculated by eq. (2.11), which gives a capture efficiency of74.6 %.

Ω = 4π sin2(ω

4

)(2.10)

εcaptured = 1 − 2 sin2

arcsin(

11.5

)2

(2.11)

(2.12)

The other extreme is a wall thickness t → R, approaching the radius R of the tube, thus forming asolid cylinder. Assuming the wavelength shifting only occurs on the surface of the cylinder, the incidentangles on the next cylinder surface will stay the same as illustrated in Fig. 2.11. Due to the proximityto the surface this will lead back to the eq. (2.10) and (2.11), resulting in the same value of 74.6 %,that where previously calculated. Therefore it can be assumed that for a tube the same result is valid.The hole in the center has no effect, because when the light leaves the tube towards the center it willalso have to enter back into it again and due to symmetry the incident angles stay the same, thus notchanging the outcome.

Figure 2.11: Illustration of total internal reflection in a cylinder cross-section (left) and illustration of light captur-ing in a glass plate.

2.2.4 WOM Development

This section gives a short overview on previous developments and basic design decisions that havealready been made. The basic work already done on the WOM with a focus on wavelength shifting andefficiency can be found in [27]. The adiabatic light guide necessary to direct the light from the WLStube to the PMTs has been developed in [31]. Some prototype pressure vessels have been producedexternally and are being evaluated in Mainz University by Markus Archinger under supervision of Lutz

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2 Theoretical Background

Koepke. The readout electronics will be a joint developed with the Gentoo IC DOM. The final choice ofPMT type has not been made yet. In Bonn the PMTs represented in Fig. A.7 have been considered. InMainz some PMTs are currently tested for noise rates at low temperatures. Current results show noiserates below 10 Hz. For best performance the WLS emission spectrum needs to match the PMT accept-ance as good as possible. Most considered PMTs have their peak efficiencies in the blue. Therefore thisthesis is focused on developing a WLS paint, shifting light above 400 nm.The outer hull has been chosen to have a diameter of 11.5 cm and a wall thickness of 1 cm, as a com-promise between price, availability and detection properties but they might change for further develop-ment stages. The diameter of the inner tube is 9 cm and its wall thickness will depend on the propertiesof the material chosen for it. The light guiding tube will have a height of 1 m and the height of thepressure vessel will match this including PMT height.Caused by the change of refraction index between quarz glas (n ≈ 1.5) to air within the detector, therewill be light reflected before even reaching the WLS. It is planed to partially compensate for that bytexturizing the inner side of the glass casing. Thereby the area is enlarged and the amount of photonsthat can passing through is increased. An illustration of the different texturizings can be seen in Fig.2.12.

ice airglassn=1.33 1.48 1.00

ice airglassn=1.33 1.48 1.00

α

Figure 2.12: Glass surface texture for improved transmittance from ice to glass to air with lenticular arrays (byPeter Falke). On the left without texturing, in the center with a triangular texturing and on the right with circulartexturing.

In the next chapter it will be looked at the dip coater necessary to apply the WLS paint an the opticalsetups necessary to physically evaluate a WLS paint and its properties.

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CHAPTER 3

Laboratory Setups

Within the scope of this thesis a lot of lab setups have been built and used. In order to get an overviewof these they are summarized and explained in this chapter.

3.1 Dip Coater

The dip coater is the central tool to apply paint on any sample with a desired velocity. It consists of sixmajor parts shown in Fig. 3.1. A holding structure with bottom plate, a threaded rod (yellow ellipse)that is driven by a step motor (blue circle). Caused by the rotation of the threaded rod the carriage(green) moves up and down. Two switches (marked in red and green1) define the upper and lowerbounds of the carriage path. They can be positioned as needed. In addition a clamp for paint containersis attached to the structure (black). The hole system is controlled by an Arduino UNO [32] (purple), asmall micro-controller equipped with a prototyping board and a rudimentary operating system (OS) thatcan be programmed in a C++ based language, in combination with an “Arduino Motor Shield” [33].The latter is mainly designed to drive ordinary DC motors and not stepper motors, therefore it has beendifficult to implement a control program which keeps the motor from skipping steps. The introductionof micro-stepping with 32 micro steps per step solved this problem. To control the velocity, the delaybetween the micro steps can be modified with an emulated serial interface via USB. The most reasonabledelays for coating are between 2 ms and 20 ms. The micro-controller used on the Arduino Uno is anATmega328 that runs with 16 MHz. Since not just the implemented program alone, but an OS is runningon the chip, it cannot be assumed, that a command uses only one cycle, but rather many cycles percommand. Because the OS is not a real time OS, constant reaction times can also not be guaranteed.This gives rise to uncertainties when calculating the dipping velocity v, therefore a measurement wasconducted and the results are shown in Table A.1.

1 behind the carriage

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3 Laboratory Setups

Figure 3.1: The dip coater, (Left) Before the wrapping with colored marks for explanation. The step motor (blue)can be seen, the casing for the turning rod (yellow), the carriage (green), the moving range limiting switches (redand green), control unit (purple) and the paint vessel holder (black). (Right) After the coater has been wrappedand equipped with the exhaust at the top left corner, for extraction of paint vapurs.18

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3.1 Dip Coater

A fit for the conversion factor m in eq. (3.1) resulted in eq. (3.2) as can be seen in Fig. 3.2. While Ddenotes the delay time given to the micro-controller.

D =mv

(3.1)

m = 18.558 ± 0.009cm ·ms

min(3.2)

With this result it is now possible to set the desired dipping velocity accurately. In order to keep theoperator save from the solvents evaporating while applying paint, the entire coater has been wrapped intransparent polyvinyl chloride (PVC) sheets and an exhaust has been installed at the very top left corner,see Fig. 3.1. To keep the paint free of dust, the coater has been placed in a clean room.

Figure 3.2: Determination of dipping velocity depending on the internal delay in the dipcoater control. A fitresulted in an inverse slope of m = 18.558 ± 0.009 min

cm·ms .

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3 Laboratory Setups

3.2 Absorption Measurement Setup

In order to quickly characterize wavelength shifting paints and without wasting expensive quartz glasstubes, a setup for absorption measurements with microscope slides has been developed. First the paintis coated on a microscope slide with the dip coater. Then the slide is put between the light source and aphotodiode (PD) for the measurement. The same measurement is repeated on a microscope slide withoutpaint for normalization. When several slides with the same paint and different known paint thicknessesare measured the absorption length can be determined as function of wavelength. A schematic of thesetup can be seen in Fig. 3.3.

Figure 3.3: Schematic setup of the absorption measurement. The arrows indicate the light path onto the photodi-odes (PDs) where the light is captured.

3.2.1 Light Source

The light used for the measurement is produced by a high pressure (70 bar) Xenon-Lamp and directedinto a monochromator that produces monochromatic light by constructive and destructive interferencewith a reflective grating. The amount of light passing through can be adjusted with slits on both inputand output. The width of the slits is also responsible for the achievable band width. Per mm openingof the slit the band width increases by about 3nm. In front of the monochromator exit, a chopper wheelis installed that interrupts the light output with adjustable frequency in order to be able to cancel outambient light on the PDs with the lock-in amplifiers. Therefore the frequency signal of the chopper is

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3.2 Absorption Measurement Setup

Figure 3.4: Light source and reference setup. The violet arrows indicate the light path. The light from themonochromator slit is projected onto the liquid crystal light guide. For reference a beam splitter In the beam pathtakes light out of the beam. The light used for reference is reduced by means of a collimator.

given to the lock-in amplifiers as reference. A more detailed description on the monochromator and thelock-in amplifiers can be found in [34]. Behind the chopper wheel, a lens with focal length of 5 cm isinstalled in order to focus the light. The setup illustrated in Fig. 3.3 is shown with the correspondinglight path on the real setup in Fig. 3.4. The light is split into two beams by a beam splitter. Thereflected beam is collimated onto the reference diode, at the same time weakening the intensity suchthat the reference photodiode is not saturated. The reference diode is read out with the first lock-inamplifier. The light that is not reflected at the beam splitter is coupled into a liquid crystal light guidefor illumination of the sample.

3.2.2 Sample Evaluation

The light exiting the liquid crystal light guide is directed towards a microscope slide that is coated witha paint sample as seen in Fig. 3.5. When the light reaches the WLS in the paint, it is absorbed andre-emitted isotropically (at a different wavelength). That means, like determined in section 2.2, that74.6 % of the converted light is captured within the slide and guided by TIR to its sides. The remaining25.4 % of the emitted light is emitted isotropically to both sides of the slide. With a PD in a distance off

≈ 5 cm and an area of 35 mm2, less than 0.5 % of the light will still reach the photodiode. This amountis within the size of the error on this measurement and can therefore be neglected. To normalize thesemeasurements to the total amount of light exiting the liquid crystal light guide, a reference measurementhas to be taken with the same setup, but without the paint. Here the light can pass through the microscopeslide unhindered and is detected in the PD that is read out with the second lock-in amplifier.

To calculate the amount of absorbed light like in Fig. 3.6, the following calculations are necessary:

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3 Laboratory Setups

Figure 3.5: Picture of a microscope slide in the absorption setup

Isample,corrected =Isample

Isample,reference(3.3)

Inormal,corrected =Inormal

Inormal,reference(3.4)

Isample,normalized =Isample,corrected

Inormal,corrected(3.5)

Iabsorbed = 1 − Isample,normalized (3.6)

In the above formulas Isample describes the light intensity exiting the slide sample, measured at the secondphotodiode and Isample,reference the corresponding intensity from the reference diode. Analogously, Inormaland Inormal,reference are intensities from the reference measurement without paint, but just a microscopeslide. thus intensity fluctuations of the the input light are corrected in the intensity ratios Isample,correctedand Inormal,corrected. Iabsorbed is the fraction of the light’s intensity that is absorbed by the sample paint.For all measurements with the lock-in amplifier, always ten samples per wavelength have been takenand averaged. The variance is also determined and propagated by the use of Gaussian error propagation.Large errors can be caused by a weak signal or strong fluctuations in the lamp’s intensity.The later oftenhappens before the lamp has reached thermal equilibrium. Therefore, a warm up time of at least 30 min.has been used.

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3.3 Determination of Emission Spectra

Figure 3.6: Absorption measurement with a microscope slide normalized to a measurement without slide. Below280 nm the absorption of the glass is too high for transmission measurements, although it has to be taken intoaccount that measuring without a slide changes the geometry of the setup. Thus Fresnel losses are not canceledby normalization in that particular measurement. A measurement with Poly(methyl methacrylate) (PMMA) filmon a microscope slide normalized to a microscope slide is shown as well. It can be seen that a thin PMMA filmwithout WLS does not increase the absorption.

3.3 Determination of Emission Spectra

In order to calculate the paints efficiency, it is necessary to know the output spectrum of a paint. Alsoin case there is re-absorption or changes in the spectrum when modifying parameters like the bindertype, it is useful to see the spectrum. In order to achieve that, a spectrometer of type “BLUE-WaveMiniature Fiber Optic Spectrometers for UV-VIS-NIR (UVIS - 50)” from StellarNet has been used[35]. For calibration it is necessary to take a dark measurement, that is a measurement taken with thesame exposure time but without any source in order to correct for dark currents2 in the pixel sensor.The dark current is corrected by taking two regular measurements where one is without source butincluding ambient light3. This measurement is called a background measurement and includes a darkmeasurement. To obtain values that are proportional to the intensity of a specific wavelength bothmeasurements are subtracted like in eq. (3.7).

Ncorrected(λ) = Nsample(λ) − Nbackground(λ) (3.7)

As these values are photon counts the error is determined by the Poisson statistics and further errorsare calculated by Gaussian error propagation. Internally the spectrometer separates the incoming light

2 A small electric current that flows through photosensitive sensors even though no photon triggered a response.3 The main light hast to be off for all measurements with the spectrometer, however some small light sources remain.

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3 Laboratory Setups

according to its wavelength with a reflective grating. The separated light can be detected with a charge-coupled device (CCD)-sensor according to its position. Therefore the photon counts per wavelengthinterval can be determined. As this sensor has a wavelength dependent photon sensitivity the measureddata has to be scaled subsequently. The required values are supplied by the manufacturer.

3.4 Efficiency Measurement Setup

Figure 3.7: Schematic setup of the efficiency measurement. The arrows indicate the direction and color change ofthe light. Within the tube wall the light propagates in a spiral fashion.

The efficiency measurement determines several combined properties. For this measurement a paintcoated transparent plastic or glass tube is illuminated and the amount of captured and guided light ismeasured at the end of the tube as shown in Fig. 3.7. The efficiency is the ratio of photons reachingthe readout PD vs. illuminating photons. Thus the efficiency includes efficiencies for entering the tube,absorption by the WLS, quantum efficiency of the WLS, light capture and light transport. This gives aestimate for a real detector application, but at the same time makes it difficult to disentangle the singleefficiencies and identify those that need to be improved upon.The basic setup for the light sources is the same as in section 3.2.1 but the sample evaluation worksdifferently. When the light leaves the liquid crystal light guide it is directed onto the coated tube in away that all of the light meets the surface of the tube. First losses due to reflection (about 4%, see alsosection 2.2.3) will occur here. Additional losses may occur by absorption of the light by the substrate

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3.4 Efficiency Measurement Setup

or non absorption of the light by the WLS. The light absorbed by the WLS is isotropically re-emitted ata higher wavelength with a later to be determined efficiency. As described in section 2.2, for an idealtube 74.6 % of that light is captured due to the tubes geometry and guided towards the sides by TIR.However, absorption of this light in the tube and by the WLS as well as scattering on impurities andsurface craggedness may decrease the efficiency further. On one end a part of the surface is contactedto a PD via optical gel to measure the light intensity. By scaling this fraction of the output surface to thewhole surface, the overall luminosity can be determined. This is only possible because the point wherethe WLS emits the shifted light is at least 20 cm4 apart from the PD. This way it is assured the light isdistributed equally when reaching the edge of the tube i.e. near field effects can be neglected. To obtainabsolute values, the measurement is normalized to a setup where the liquid crystal light guide is directlyconnected with optical gel to the PD and the intensity per wavelength is determined.It has to be taken into account that the PD has wavelength dependent sensitivities, so the followingcalculations have to be performed to obtain the efficiency ε(λ)

ε(λ) =Nout(λ)Nin(λ)

C(λ) · εΩ, (3.8)

where Nin(λ) and Nout(λ) are the measured photon rate entering and exiting the tube respectively. εΩis a correction factor for the tubes geometry and C(λ) is a wavelength dependent calibration factor forlamp fluctuations between the calibration and the sample measurement. The in coming photon rate canbe obtained by the following equation.

Nin =Inormal

hcλ · R(λ)

, (3.9)

where R(λ) is the response function of the photodiode taken from the data sheet [36]. For the outgoinglight, which is not monochromatic, the response function has to be folded with the to one normalizedoutput spectrum S WLS (λ) of the WLS (see section 3.3) to obtain an average value:

〈R · Eγ〉 =

∫S WLS (λ) · R(λ)

hcλ

dλ, (3.10)

so for Nout the following equation can be obtained:

Nout =Isample

〈R · Eγ〉. (3.11)

The calibration factorC(λ) =

Inormal,reference

Isample,reference, (3.12)

cancels out the fluctuation between the sample and normalization measurement by dividing by the in-tensity from the reference diode for each measurement. In an ideal case this factor would always be one.The correction factor

εΩ = 2 ·Aoutputarea

Acoveredoutputarea, (3.13)

has been introduced to correct for the small PD size that covers the tube end only partially and only onone side. The calculation for the covered area assumes a very accurate positioning of the PD. This is inreality not always possible therefore an error of about 4% for this value should be taken into account.

4 A results of the calculation described in section 3.5 for tubes with 2 cm diameter

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3 Laboratory Setups

However, there is an additional error, when moving the PD to a different position at the end of the tube.While the covered area should be within 4 % constant, the values may fluctuate by about 15 %. It wasfirst assumed that this is due to geometric effects causing in-homogeneous light distribution. But as theresults of the calculation described in section 3.5, this is not the cause for tubes longer than 20 cm. Thesefluctuations can also occur when the PD is kept in place and just the tube is turned along its symmetryaxis. This leads to the assumption that it is caused by a difference in losses due to tube or surface quality.This is not a problem for the approximate determination of the ideal paint or the general shape of thespectra. It does however become a problem when one tries to fine tune the ideal paint. Therefore it willbe necessary to understand the losses better.

Figure 3.8: Top view on the measurment setup with a PMMA tube sample coated with a PMMA based Bis-MSB(WLS) paint at the right side. The paint is illuminated by the liquid crystal light guide with 360 nm light and readout with a photodiode at the left end. The total length of the tube is 28 cm. On the right side is a black plasticplate connected to the tube by optical gel in order to absorb the light at that end. However it is clearly visible thatnot all the light is absorbed.

3.5 Position Depending Efficiency

To investigate the efficiency as a function of tube length, tubes with different lengths have been coatedat the bottom and the efficiency was evaluated with the setup from section 3.4. However, due to theuncertainty introduced by the paint surface quality and / or absorption in the substrate, those resultsshowed low accuracy. In order to examine this and obtain more reliable results, the behavior on asingle tube was investigated. A basic setup like in section 3.4 is used. In addition, a X-Y-Z scanneris used to move the liquid crystal light guide along the tube and change the point where the WLS isexcited (Fig. 3.9). In contrast to section 3.4, only one input wavelength is used and the normalizationis handed differently. All corrections described in eq. (3.8) are applied, excluding the correction factorεΩ. As the light emission point comes into proximity (closer than about 20 cm) of the photodiode in thismeasurement, the light emission on the pipe edge will no longer be homogeneous. To compensate forthis effect a calculation (assuming no absorption) has been done. The basic principle of this is illustratedin Fig 3.10.The blue square in the center represents the wall of the tube, unfolded to a plane. In reality, it istheoretically possible for light to circle endlessly around, therefore periodic boundary conditions arechosen and the tube keeps repeating itself. At the top, a PD is coupled to the tube with a defined widthw. The PD is movable in y coordinate, as it is not always on a vertical line to the light source. In theprogram, the light source is approximated to be point-like and can change its position in x direction.

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3.5 Position Depending Efficiency

From the point source, light rays with 10, 000 angles between 0 to 180 are propagated and it is testedwhether they are detected in the PD(s) or not. For the far field, the result of #hit

10,000 approaches wd . This

is not true for the near-field, where a const.x distribution is observed for photodiode and light source in

one line on the x axis. This calculation allows to compensate for these effects in the real measurement,when the same geometric circumstances are simulated.The computed results (Fig. 3.11) are based entirely on geometric factors. For values of x >> 20 cm thesimulation results approach5 εΩ

26. By dividing the obtained efficiency ε(x) according to eq. (3.14) by the

computed results fsim(x), the data is normalized and corrected for geometric dependencies. The factorof 2 arises from the fact that both tube ends are taken into account.

εnorm(x) = 2ε(x)

fsim(x)(3.14)

It should be mentioned that it is also possible to include the angular acceptance of the PD in thesimulation. The effects of this consideration can also be seen in Fig. 3.11. But it is not yet used fornormalization, because the angular acceptance of the PD for angles close to 90 is very low, so thecontribution of these light rays would be neglected. But especially for those rays, the probability forscattering or absorption is highest, because their path is the longest. To correct for the PD angularacceptance a absorption parameter, which is currently unknown, needs to be included as well. However,this correction becomes redundant as soon as the readout is done with PMTs on the hole tube end.

Figure 3.9: Schematic setup for a measurement to determine losses with positional dependence.

5 With an error of ≈ 4 % depending on the wall thickness of the tube6 εΩ from eq. (3.13)

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3 Laboratory Setups

Figure 3.10: Schematic sketch for the working principle of the geometric transmission calculation.

Figure 3.11: Results of the geometric dependence calculations for d = 2π cm and w = 0.6 cm. The first twocalculations (black, blue) are just based on plain geometry. The second two (red, yellow) also include the angularacceptance of the PD (see Fig. A.6). Based on this calculation it is also possible to simulate different absorptionlengths and compare those to the measured ones. However a qualitative analysis of this has not been done yet.

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3.6 WLS Quantum Efficiency Measurement Setup

3.6 WLS Quantum Efficiency Measurement Setup

It is of interest to not just determine the overall efficiency of the detector, but also the efficiency ofall participating parts contributing to the overall efficiency in order to find which parts need or allowfor improvements. One of the parts which are very important is the WLS, or more specific the WLSquantum efficiency within the plastic binder. In the measurement described in section 3.4 only thecombined light guiding efficiency and absorption in the light guide is measured. In order to avoidlight guiding efficiency and only allow a minimal amount of absorption, the light not captured by TIRin the light guide is most appropriate. In order to obtain an absolute measure of the efficiency, thegeometry of the setup must be very well known or a well known reference sample must be used fornormalization. Such a well known reference is a Lambertian reflector (spectralon) [37], which willbe used for normalization. The setup built to perform such a measurement is depicted in Fig. 3.12.The incoming light is being projected by a lens and aimed at the target under the angle α. During themeasurements, this angle is set to 35± 1 , with the goal to avoid reflection in the direction of the signalPD. As target a WLS paint coated microscope slide that re-emits the light or a spectralon block thatscatters the light with the same7 angular distribution like the coated slide is used. The light from thetarget is projected by a lens onto the signal PD. The angle β of the signal PD and the lens towards thetarget is always 0 ± 1 8.

Figure 3.12: Schematic setup of the WLS quantum efficiency measurement. Light is projected onto a WLS paintcoated microscope slide and the emission is measured. For normalization, the reflected light from the Lambertianreflector spectralon is used. For confirmation that the WLS emits Lambertian too, the arm with Signal PD andcorresponding lens can be rotated by the angle β. The light source and reference setup is the same as before.

For sample evaluation the emission intensity of the WLS paint coated slide and the reflection spec-7 To be shown8 Meaning the way it is shown in the Fig. 3.12. A upwards pointing PD would have −90

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3 Laboratory Setups

trum of a spectralon sample are taken. Then the resulting WLS spectrum is normalized according to eq.(3.15) which is based on eq. (3.8) with the spectralon sample as normalization. NWLS(λ) is obtained ac-cording to eq. (3.11) and NSpectralon(λ) according to eq. (3.9). C(λ) is calculated from the correspondingreferences as before.

ε(λ) =NWLS(λ)

NSpectralon(λ)C(λ) · ε′′Ω (3.15)

The lens in front of the signal PD is not UV transparent, meaning only a limited amount of light inthe UV-spectrum can pass through the lens. To treat the wavelengths passing through the lens as equalthe wavelength dependent transmission of the lens needs to be compensated, for that ε′′

Ωis introduced in

eq. (3.15). To obtain it, the transmission spectrum of the lens had to be taken. For this the known lampand monochromator setup was used and the light guide directed towards the lens with with a distanceof twice the focal length of the lens. The PD was placed in the same distance on the opposite side ofthe lens in order to project the light from the fiber onto the PD. The values of the resulting intensityILens uncorrected(λ) is then corrected by the intensity without lens IWithout Lens(λ) and the correspondingreferences IWithout Lens reference(λ) and ILens uncorrected reference(λ), according to eq. (3.16) (Fig. A.4). Tomake both, the reflection and the emission spectra, comparable the transmission spectrum of the lensneeds to be expressed in photon rates. For the wavelengths of the spectralon reflection this is doneby eq. (3.18). For the WLS emission spectrum, the WLS output spectrum normalized to one, the PDsensitivity and the lens transmission are convoluted like described by eq. (3.19). By dividing both ratesaccording to eq. (3.17) the light transmissivity of the lens for the WLS shifted light is normalized toone. This scales the transmissivity of the non-shifted reflected light from the spectralon normalizationaccordingly. Effectively compensating the spectral behavior of the lens.

ILens(λ) =ILens uncorrected(λ) · IWithout Lens reference(λ)IWithout Lens(λ) · ILens uncorrected reference(λ)

(3.16)

ε′′Ω(λ) =NUV(λ)Nvis(λ)

c (3.17)

NUV(λ) =ILens(λ)hcλ · R(λ)

(3.18)

Nvis(λ) =

∫ILens(λ) · S WLS (λ)

hcλ · R(λ)

dλ (3.19)

One other correction factor c is included in eq. (3.17), in order to compensate for the light capturedin the microscope slide, the fact that spectralon scatters the light in 2π but the WLS does emit in 4π andfor reflection losses. The capture efficiency can be calculated from eq. (2.11) to be εcaptured = 74.6 %.Thus giving a correction factor as shown in eq. (3.20) including Fresnel losses R(θ = 37) = 4.66 %from eq. (2.9) and resulting in c = 8.18.

c =2

(1 − εcaptured) · (1 − R(θ = 37))(3.20)

For reference spectralon has been used due to its properties as Lambertian reflector, but it remainsto be shown that the output of the WLS paint coated slide shows the same angular behavior. The lightfrom the WLS is emitted isotropically and part of the light is captured via TIR, but the remaining light

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3.6 WLS Quantum Efficiency Measurement Setup

is refracted according to Snell’s law while leaving the glass. It is unclear if the reflection due to theFresnel equations at high angles causes additional losses because the light is reflected multiple times.Therefore, the same setup from Fig. 3.12 is used to determine whether both samples, spectralon and theWLS paint coated slide exhibit the same reflection or respectively emission angle spectrum. In the setupthe arm with lens and PD can be moved along the angle β and therefore a measurement of the intensityat different angles with the same wavelength and light intensity from the source has been conducted forspectralon and the WLS slide. The results are shown in Fig. 3.13. As to be expected the results forthe WLS have a lower intensity but also a reflection peak for the WLS can be seen. In order to correctfor that peak, the measurement has been repeated with a non-coated glass slide and the result is alsoshown in Fig. 3.13. As to be seen in Fig. 3.14 it has been tried to remove this peak by fitting the plainglass measurement to it. This was not successful9. However, while the functions are scaled to one atβ = 0, it can be seen that the function shapes outside the peak region match within a reasonable error.Unfortunately the DAQ of this setup does not provide an estimate on the error.

Figure 3.13: The intensity distribution obained in the angular measurement of spectralon, a WLS paint coatedmicroscope slide and a plain microscope slide. The proportions between the measurements are as shown.

9 It looks like a angular offset, but even modifications by hand only change the shape but never compensate it fully.

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3 Laboratory Setups

Figure 3.14: Scaled data from Fig. 3.13 with the attempt to correct for the reflection peak of the WLS paint coatedslide using the plain measurement of a microscope slide. At angles close to −90 the intensity of the from theWLS emitted light is very low. Thus it can be assumed that the error has a size comparable to the difference to thespectralon intensity. Therefor, it is assumed that both functions match.

3.7 Camera Measurement

In order to determine the quality of the used substrates and paint surfaces and to find possible sourcesof losses, pictures with a digital camera of those samples have been taken. Exposure times of severalseconds while illuminating the samples with light were used. When a substrate only e.g. glass orplastic tube was examined they where illuminated at one end with an integrating sphere and white light.If a painted surface was analyzed the paint was illuminated by the liquid crystal light guide with awavelength matching the maximum of absorption of the used WLS. These measurements give a roughindication of material quality and possible losses. Quantitative measurements are not possible yet. Butit is planned to calibrate and analyze these pictures to gain further knowledge on the material propertiesand ways to improve them.

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CHAPTER 4

Commercial WLS Paint

4.1 Characterization

To test the basic functionality of the wavelength shifting and light guiding tube concept, a commercialWLS paint [38] and quartz glass tubes with a diameter of 2 cm have been tested previous to this thesis[27]. After these promising first results the paint needed to be characterized further.

4.1.1 Paint Layer Thickness

In order to understand the paint behavior for different coating velocities the paint layer thickness needsto be determined. It was observed, that several days after the glass tubes had been coated with thepaint, it started to become opaque and peel off as shown in Fig. 4.7. The peeling of the paint hasbeen thoroughly investigated and is described in section 4.2. However, the peeling has been useful toinvestigate the paints thickness. For different coating velocities, four samples on microscope slides havebeen produced each. The upper and lower ends have been covered with sticky tape for well definedborders and to avoid varnish tears. After the paint had dried, the sticky tape has been removed and theremaining layers of paint where taken off carefully, which succeeded in most cases. These paint filmshave been measured in width, length and weight. In combination with the density (1.05 g/mL at 25°C) of poly-vinyltoluene (PVT) [39], the binder of the commercial paint [38], the thickness could bedetermined. This assumes that the unknown amount of WLS in this paint is negligible. Comparing toa similar paint described in [40] and the conditions described in section 2.2.2 that are necessary for theWLS not to crystallize, this assumption seems justified.1 The results of this measurement can be seenin Fig. 4.1. They show reproducibility and a monotonic increase of thickness with the coating velocity.In literature [41] often a h ∝ vx behavior is assumed for the layer thickness in dip coating techniques,with h being the thickness, v the velocity and x a parameter to be fitted. However, this does not describethe results here, as a saturation effect for high velocities is observed. In order to compensate for the

1 It has also been observed that, when the weight was measured one day after the coating and three days later, the weightwould decrease by about 1 − 2 %. This is probably due to the fact that not all of the solvent (Toluene) evaporates within thefirst 24 h.

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4 Commercial WLS Paint

saturation a divisor v + 1[

cmmin

]has been introduced in

h =avx

v + 1 cmmin

. (4.1)

This equation is purely empirical, as for velocities higher than the tested ones a decrease in thicknesswould be predicted. It is certain that in a function to determine the layer thickness from first principles,gravity, paint viscosity, evaporation rate and coating velocity should be primary parameters. The curve-fit of eq. 4.1 to the data (shown in Fig. 4.1), determined a = (0.0281.0009) mm and x = 0.90.02 with aχ2/d.o.f= 0.957.

Figure 4.1: Reproducability of paint layer thickness as a function of the dip coater velocity and the an empiricalfit to this correlation. It should be pointed out that these values are averaged over the paint surface and do notassure even paint distribution.

4.1.2 Absorption Measurements

The next step in the characterization is the determination of absorption in dependence of the paintthickness and its reproducability. In order to do this a setup like described in section 3.2 for absorptionmeasurements has been used. A set of microscope slides with different coating velocities for everysecond slide has been produced. Absorption has been measured for all slides, the results are shown inFig. 4.2. These results are consistent with the results from [27] and the expected capture efficiency fromsection 2.2 when the light guiding and wavelength conversion efficiency is beyond 90 %.

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4.1 Characterization

Figure 4.2: Absorption measurements for different paint thicknesses. The black dashed lines are a second line ofsamples with the same coating velocity to test reproducability of these measurement technique. Except for the0.02 mm measurement, all fit well within their error. The reason for the difference with the 0.02 mm sample isunclear. However due to the very small discrepancy it was not given much attention and might just be caused bya slightly tilted slide.

The combination of these results allows to determine the mean free path for the commercial paint independence of the wavelength. Therefore the absorption is plotted against the paint thickness for everymeasured wavelength and a exponential function, eq. (4.2), is fitted to it.

Iabsorbed = 1 − exp−h+aλ (4.2)

Here λ is the mean free path and a a correction factor for the thickness. a is usually below 10 % of thethickness h but improves the χ2/d.o.f significantly. Due to the introduction of a and the normalizationof the absorption, there is no scaling factor necessary. An example of this fit at 375 nm can be seen inFig. 4.3.Applying this procedure to all wavelengths and plotting the results of λ including its error results inFig. 4.4. To achieve sufficient absorption, a paint thickness of several mean free paths is needed.Since the maximum thickness for one layer is about 0.02 mm, this seems in probable to accomplishfor wavelengths below 330 nm. The supplier advertises the possibility of several layers of paint but inan attempt with multiple layers the surface quality and adhesiveness of the paint suffered dramatically,therefore investigations in this direction where discontinued.

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4 Commercial WLS Paint

Figure 4.3: Exemple fit for the mean free path (eq. (4.2)) at 375 nm with the values obtained from Fig. 4.2.Producing the following results: a = 0.42.08 µm, λ = 8.33.08 µm and χ2/d.o.f= 26.7. The rather large χ2/d.o.f isprobably du to the not included error of the thickness.

Figure 4.4: Collection of all fitted mean free path results as shown in Fig. 4.3. For comparison, 0.02 mm is aboutthe maximum reachable paint thickness.

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4.2 Problems

4.1.3 Efficiency

For full characterization the efficiency (section 3.4) and the spectral output (section 3.3) of the WLSpaint have also been measured as shown in Fig. 4.5.2 It can be seen that emission and absorptionspectrum are well separated, i.e. the paint has a large Stokes-shift (section 2.2.2). The reason forthe discrepancy between the expected efficiency 74.6 % (section 2.2) and the one seen in Fig. 4.5 isunclear3 Even when the quantum efficiency of at least 90 % stated by the manufacturer and losses dueto reflection of about 4 % due to Fresnel reflection (section 2.2.3) are taken into account, there are stilllosses of about 36 % unaccounted for. At this point the most reasonable explanation are losses in thelight guiding process within the tube, such as scattering and absorption. This will be further investigatedin section 5.3.

Figure 4.5: Efficiency of commercial paint and the corresponding emission spectrum, measured on a 30 cmPMMA tube with 2 cm diameter and 2 mm wall thickness. The paint layer is 0.02 mm thick, according to themeasurements in Fig. 4.1. The slight variation between the shapes from the absorption spectra in Fig. 4.2 isprobably caused by the change of refraction index with wavelength of the PMMA.

4.2 Problems

As already previously mentioned there have been some problems with the paint. Most importantly thefact that the paint starts to peel from glass surfaces and the efficiency being more than a factor two lower

2 During the characterization of the commercial paint first attempts to develop a custom-made WLS paint (chapter 5) havebeen made. In this process the program for the evaluation of the efficiency measurement was rewritten as the previousversion had many properties of the commercial paint been embedded in the script. While doing so it turned out that aintegration routine of the old script aborted its iteration too early and caused results to be about a factor of three to high.Due to that reason the results in Fig. 4.5 differ from the results presented in [27].

3 One would expect to see 74.6 % of the values measured in Fig. 4.2 for the 0.02 mm slide(s).

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4 Commercial WLS Paint

than expected, but also the paint turning opaque after some time. Several approaches to find the causesand solutions to this problems have been undertaken.The reason for the paint turning opaque has been determined to be the optical gel used to opticallyconnect the tube with the PD and the fiber, where the later connection has been discontinued for morerealistic measurement conditions. The first cause for the low efficiency is the low absorbance of thepaint below 330 nm. This causes the capture efficiency in this regime to be lower than what one wouldhope for. Especially with the spectrum of the Cherenkov light being proportional to 1

λ2 as shown insection 2.1.1. To improve the capture efficiency in this wavelength regime it has been tried to addadditional WLS with a well suited spectrum. Such a WLS is p-terphenyl (PT). Its absorption maximumis at 275 nm and its fluorescence maximum at 339 nm [42], very close to the absorption maximum ofthe commercial WLS paint. In order to add PT to the paint, 100 ml have been heated to about 110C4

and 0.892 g PT where added. The approximate amount of PT to use was estimated from the amountsused in [40]. However, after 24 hours a part of the PT had crystallized. As the crystals had sunk to theground it was still possible to coat a sample and evaluate the results as seen in Fig. 4.6. An improvementis clearly visible, but with the limited solvability of PT in the paint, it is rather limited. Also, as onlyabsorption has been measured here it is not clear whether this can be realized in efficiency, too. Theshift of almost 40 nm of the absorption peak compared to literature [42] can be noted and is possiblycaused by the embedding of the WLS into the plastic matrix. Due to the absorption of the microscopeslides it is not possible to observe the absorption at lower wavelength.

Figure 4.6: Absorption spectrum of the commercial paint with additional P-therphenyl (PT) solved in maximumsolution and the paint without additives for comparison. (Due to fluctuations the value at 305 nm has been re-moved.) It is clear that some improvements can be made this way, but due to limited dissolvability the improve-ments are limited as well. The large errors compared to Fig. 4.2 are caused by a lower light intensity in thismeasurement.

4 Adjusted temperature of the heating plate, not the paint

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4.2 Problems

Sample Treatment Thickness [mm] Time [d]Untreated sample 0.02(1) 25Untreated sample 0.014(9) > 120Cleaned with ethanol and isopropyl 0.02(1) 30 ± 3Cleaned with ethanol and isopropyl (sample 1) 0.014(9) > 120Cleaned with ethanol and isopropyl (sample 2) 0.014(9) 35Stored in low humidity 0.02(1) 33Stored in low humidity 0.014(9) > 120Cleaned with Mocasol [43] 0.02(1) 19.5 ± 7.5Cleaned with Mocasol (sample 1) 0.014(9) 12Cleaned with Mocasol (sample 2) 0.014(9) > 120Stored in a freezer (≈ −20C) 0.02(1) 1Stored in a freezer (≈ −20C) 0.014(9) 2Cleaned with Piranha Solution [44] 0.02(1) 49Cleaned with Piranha Solution [44] 0.014(9) 49Cleaned with a UV-Light and Ozone cleaner (30 min) 0.02(1) 25Cleaned with a UV-Light and Ozone cleaner (30 min) (sample 1) 0.014(9) 25Cleaned with a UV-Light and Ozone cleaner (30 min) (sample 1) 0.014(9) > 120With a mono layer HDMS applied (adhesion promoter [45]) 0.02(1) 26 ± 3With a mono layer HDMS applied (adhesion promoter [45]) 0.014(9) 15.5 ± 3.5

Table 4.1: Results of adhesion tests with commercial paint on microscope slides. Different treatments have beenapplied with each two samples at two different thicknesses. The time between coating and first signs of paintpeeling has been observed. All samples except the untreated one are cleaned with ethanol and isopropyl beforetheir individual treatment. For samples with peeling times > 120 no peeling has been observed yet. For all othertimes the mean of the two values has been obtained with its corresponding difference when not both samplesstarted peeling at the same time. However the times between observations have been bigger on occasions. Usuallya error of five days has to be considered. Only in the first five days it was observed daily.

The unexpectedly low general efficiency has also been observed in other paints and will be discussedin section 5.3. For the peeling problem as shown in Fig. 4.7, three different reasons seem probable.

1. Grease and other residues from the production process of the glass are left on the surface.

2. The thermal expansion coefficient difference is to high that it can not be compensated by theflexibility of the plastic paint.

3. The air humidity might damage the paint.

In order to test for those, different samples have been produced and it was observed if and when the paintstarted to peel off. The results of these observations are shown in Table 4.1. The different treatments doonly seem to influence the time until the paint peels of in the order of a few days. Most significant is thedependence of the peeling time on the paint thickness. Therefore the most reasonable explanation forthe peeling is the difference in thermal expansion coefficient and the higher elasticity of thinner paintfilms. However when the paint film has a low thickness, no reasonable amount of absorption can beachieved.

It has also been tried to use PMMA tubes as substrate for the paint. In these cases the paint did notpeel, giving another indication that the difference in thermal expansion coefficient is the reason for the

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4 Commercial WLS Paint

peeling. PMMA has the additional advantage that the plastic tube might be solved partly at the surface,thus allowing for a better bond between paint and substrate.

Figure 4.7: A Picture showing the peeling of the paint on a microscope slide (left) and a opaque paint layer onquartz glass tube (right)

4.3 Results

The commercial paint allowed to show the feasibility of the wavelength shifting paint concept and gavevaluable insight into properties of WLS paints. But it also has several shortcomings like bad adhesiveproperties for the desired paint thickness and a long and non-constant mean free path in the wavelengthregime of interest. To use PMMA as substrate allowed good adhesiveness, but PMMA as substratebrings new and different challenges like stress-cracks (see section 5.2). While the parallel to this con-ducted development of customized WLS paints showed first successes (chapter 5), the investigations onthe commercial WLS paint where abandoned for the time being in favour of the customized paint.

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CHAPTER 5

Development of Wavelength Shifting WaveGuiding Tube

5.1 Development of Wavelength Shifting Paint

5.1.1 First Approaches

The first recipe for paint production and composition were obtained from [40]. It foresees 100 mldichloromethane, 1.48 g PT and 2.37 g paraloid (B44) to be mixed, while being heated. However onlyparaloid was stated as ingredient in [40], while this is only a brand name encompassing several typesof plastic that were not specified. After one hour not all of the paraloid B44 had been solved and anadditional 50 ml of dichloromethane was added. With this modification it was possible to create the firstpaint, while at the same time the insolvability of paraloid B44 indicated that it is not the paraloid usedin [40]. Coating this paint on microscope slides with different velocities with the diper coater (section3.1) created a opaque paint surface. Measuring these samples with the absorption setup (section 3.2)only caused a very noisy measurement as it is expected from a diffuse scatterer. There was no indicationof additional absorption by the WLS. The milky surface was described in [40] as well. However, theirapplication might benefit from that, which is not the case for light guiding as needed here. In [40] thechoice for dichloromethane is substantiated by its high vapour pressure and low boiling point, one mightspeculated that the very same factors cause the milky surface. Hence tests with different solvents likeanisole, toluene and xylene have been made. Though this mixture improved the surface quality, it wasstill not considered usable for light guiding. Also in some cases the solvent did not solve PT in sufficientamounts. A sample of a PMMA / anisole solution from [46] was obtained which produced a very smoothsurface with high transparency on first attempt (shown in Fig. 5.1). Due to the very high price of thissolution and its unknown composition it was decided to attempt to reproduce it. While the equipmentand amounts of WLS for very accurate tests of solvability are missing, a few rather intuitively preparedpaint compositions yielded very useful results. 15.40 g PMMA granular were dissolved in 120 ml ofanisole while slowly being heated up to 180 C1 under constant stirring. Once the PMMA is solved0.46 g of PT were added. The result of evaluating this paint with WLS in terms of absorption can beseen in Fig. 5.1. The pure PMMA sample has no additional absorption, making it a perfect candidatefor a basic paint. The sample with added PT sows a well defined absorption spectrum caused by the

1 180 C is the temperature dailed at the heat magnetic stirrer [47], the temperature of the liquid itself is about ≈ 68 C

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5 Development of Wavelength Shifting Wave Guiding Tube

WLS.

Figure 5.1: Absorption of a PMMA paint with and without PT. Due to the low solvability of PT in anisole not allof the light can be absorbed.

During the study of the commercial paint in terms of adhesiveness the custom made paint has beensampled, too. The paint based on PMMA and anisole was tested with the same coating velocities usedfor the commercial paint. However this does not ensure the same thickness due to different evaporationand viscosity properties. The surface quality was good and no signs of peeling have been observed yet.2

The PMMA paint does not to peel of from the microscope slides, the reason for that is assumed to bethe higher elasticity of PMMA in comparison to PVT. Also the paint layer is probably thinner due toan apparent lower viscosity of the paint. With this basic knowledge on how to create a WLS paint withgood adhesiveness and a smooth surface, the paint parameters could be investigated in more detail.

5.1.2 Solvents

For the paint development as described in section 2.2.2 any solvent that solves both binder and WLScan be used. For the WOM detector paint development the following solvents have been put to con-sideration, because they have been found to be used in similar applications: Anisole, Dichloromethane,P-Xylene and Toluene. For most parts of the paint development anisole has been used because dissolvesPMMA and most of the WLS available. It was the least poisonous of the four solvents. However exper-ience were made with all solvents and the for further development important ones will be presented inthis section. Several solvability tests have been conducted with PT that are presented in Table. 5.1. The

2 One exception has to be made. A sample of the PMMA paint has been kept in the open until the viscosity due to solventevaporation was approximate as high than the one of glycerin. With slow coating it was possible to create a layer with anestimated thickness of ≈ 0.5 mm. This layer peeled off within five days.

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5.1 Development of Wavelength Shifting Paint

Solvent Gram of PT ResultToluene 1.26 Not solvedToluene 0.33 SolvedP-Xylene 1.17 Solved at 130 C but fallen out at 70 CP-Xylene 0.26 SolvedAnisole 0.66 Solved but fallen out after 24 hAnisole 0.25 SolvedDichloromethane 0.50 Solved

Table 5.1: Results from solvability tests with each based on 50 ml of the named solvent and PT. All samples wereheated to temperatures close to the boiling point of the solvent in order to dissolve PT.

other important WLS like Bis-MSB and PMP seem to be solvable in amounts sufficient for maximumabsorption in any of the previously listed solvents.

In general dichloromethane seems the most aggressive one, it has dissolved any plastic or WLS yetand evaporates very quickly. This also brings disadvantages, even with closed lid dichloromethaneevaporates from paints thus changing concentration with time. This often creates plastic skins on thepaint surfaces. With some plastics the fast evaporation leads to milky surfaces as described in section5.1.1. Also when coating dichloromethane paints on PMMA a significant amount of the tube surface isdissolved in the paint thus changing paint concentration relations, too.Anisole solves most plastics in reasonable amounts, if high plastic concentrations are desired it takes avery long time to solve. It is easier to solve smaller amounts and allow parts of the solvent to evaporate.The solvability of WLS in anisole will be discussed in section 5.1.4 and is displayed in Table 5.2. Thelow toxicity of anisole and its general good performance made it ideal for most of the test described inthe following sections.P-Xylene has not been investigated as much as the other solvents but it has been the impression of theauthor that it behaves very similar to anisole while being a greater health threat.Toluene has just recently become more interesting as it solves greater amounts of PT than anisole doesand it can be used to solve Paraloid B72. It has not been investigated before as it solves only approx-imately ≈ 2 g

100ml of PMMA. For the future it might be interesting to examine the paint surfaces independence of the solvents, as a fast evaporating solvent might leave a microscopically uneven surface.In turn the time until all of the residual solvent in a paint is evaporated for slow evaporating solvents isestimated3 to be several weeks. This could change the paints properties after it has been evaluated.

5.1.3 Binder (Plastic)

For paint production the plastics Paraloid B72, B44, B67, B82 and PMMA have been available. Theoptical properties of Paraloid B44 and B67 are not known and it was neither possible to produce clearnor flat paint surfaces with these plastics therefore their usage was limited to the early stages of paintdevelopment.Paraloid B72 has been used for similar purposes [48] and has therefore been considered suitable asmeasurements later confirmed.Paraloid B82 is a PMMA, however the Poly-MMA chains can have different length or the material canbe a combination of different chains lengths. Withal it has been observed that Paraloid B82 shows a

3 This is based on the fact that by heating up paint that has been drying for a week, the smell of solvent (in that case anisole)could still be sensed.

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5 Development of Wavelength Shifting Wave Guiding Tube

lower viscosity in anisole than the same amount of the PMMA bought from ALBIS PLASTIC GMBHunder the name “ DIAKON MG102D ” which is supposed to be PMMA without any additives. Itstransparency in the UV-wavelength range has been shown in Fig. 3.6. For the evaluation of WLS asdescribed in the next section the DIAKON MG102D PMMA has been use. It has been available in vastquantities and shows good film surfaces in combination with anisole.As part of the energy in the wavelength shifting process is carried away as lattice vibrations it can beassumed that the WLS efficiency is correlated with the binder used. Although this has not been observedyet. However due to the limited amount of binder types tested, this is still not excluded. Therefore itcould be interesting to see the efficiency for PVT or polystyrene which both show the necessary UV-light transparency [49]. However PVT is unlikely to be used as this is the binder of the commercialpaint which showed bad adhesiveness.

5.1.4 Wavelength Shifter

With a working mixture of binder and solvent4 the most important part has been to investigate differentWLS in order to cover the desired Wavelength range most efficient. The first step towards this directionhas been to investigate what WLS are suitable for this wavelength regime and also solvable in Anisole.A lot of (old) WLS could be acquired from DESY (Deutsches Elektronen SYncrotron) and a consideredchoice, with respect to their wavelength [42] has been tested for their dissolvability in anisole. Asshown in Table 5.2, except of PQP all WLS can be dissolved in anisole. Other WLS like PPO andPOPOP might be interesting too, but have not been available for testing. The fact that the samples arevery old, might be an advantage. Possible aging effects that decrease the efficiency with time are a notdesired and may have occurred already.

Amount [g] WLS Result0.171 PT Solved0.100 QUI (3,5,3””,5””-Tetra-t-butyl-p-quinquephenyl) Solved0.078 TPB (1,1,4,4-Tetrapenyl-1,3-butadiene) Solved0.120 Bis-MSB (p-Bis(o-methylstyryl)-benzene) Solved0.110 PQP (p-Quarterphenyl) Not solved0.070 Butyl-PBD (2-(4-Biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazol) Solved0.085 TMI (2,5,2””,5””-Tetramethyl-p-quinquephenyl) Solved0.100 PBD (2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazol) Solved0.067 BBD (2,5-Bis-(4-biphenylyl)-1,3,4-oxadiazol) Solved0.184 PMP (1-phenyl-e-mesityl-2-pyrazoline) Solved

Table 5.2: Result of a solvability test of different WLS in 40 ml of anisole. The weights are known to an accuracyof at least 10−4g therefore no errors are shown.

With this promising results paints based on the the same PMMA Anisole mixture and different WLShave been prepared: 800 ml anisole and 186 g PMMA split into nine 100 ml samples. The ratio ofplastic to solvent has been almost a factor of two higher than in previous paints, in order to increasethe viscosity and thereby the layer thickness. All paints have been coated on microscope slides with avelocity of 9.2(8) cm

min and than evaluated in therms of absorption. The results are presented in Fig. 5.2.All WLS show a desirable absorption close to one in their specific wavelength range, however Bis-MSBhas compared to it’s weight a very high photon cross section e.g. a low absorption length. This might

4 20 − 25 g of DIAKON MG102D per 100 ml anisole.

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5.1 Development of Wavelength Shifting Paint

be very useful for thin paint films.The emission spectra are arbitrarily scaled for comparison. Difference in brightness was observable byeye and is also represented in the relative error size. However, the relative brightness also depends onthe excitation intensity which has a dependence on the incoming wavelength. The incoming wavelengthhas been adjusted to the absorption maximum of the individual WLS, but at lower wavelength the lampintensity drastically reduces. In addition the sensitivity of the spectrometer might not be the same forall wavelength. Even though it is corrected for that, a lower count rate still increases the error size.

Figure 5.2: Absorption of different WLS in the same PMMA / anisole paint in absolute values (in blue on the left).And the corresponding relative emission spectra (in red on the right). For some WLS the error is bigger than themeasured values due to the high signal to noise ratio of the spectrometer for low intensities (section 3.3). It mightbe a first indication for a low wavelength shifting efficiency.

The efficiency of all paint has been measured with the setup described in section 3.4, the results areshown in Fig. 5.3. PMMA tubes with 20 mm diameter and a wall thickness of 2 mm have been used assubstrate. However the tubes showed occasional cracks after the coating. when trying to avoid those inthe direct light path they did not seem to have an effect on the results. However the efficiency of PT hasnot been measured in that charge caused by a lack of PMMA tubes at that point.

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5 Development of Wavelength Shifting Wave Guiding Tube

Figure 5.3: Efficiency of different WLS in the same solvent/binder combination and the corresponding relativeemission spectra. The errors on Butyl-PBD are extremely high due to the folding with the emission spectrumwhich is very poorly known. PT was not measured here due to a lack of PMMA tubes, but at a later point (seeFig. A.5).

PMP, Bis-MSB and TPB have the best peak efficiencies and that those seem to be limited like the onefrom the commercial paint at about 23 %. The reason for that needs further investigation and will beaddressed in section 5.3. However TPB has been reported to deteriorate with time [48], which rendersit useless for usage in a long term detection unit.With its very high photon cross section, the widest wavelength range and in comparison high efficiencyBis-MSB seems ideal. In order to investigate the relation of concentration and paint layer thickness aswell as weather there are additional losses due to re-absorption for very high WLS concentrations. Ameasurement with different Bis-MSB concentrations and coating velocities has been conducted and isshown in Fig. 5.4. While [48] indicates that the previously tested concentrations of Bis-MSB are alreadyvery high, only lower concentrations have been tested. The change in concentration of a factor threehas a stronger effect than a change in velocity of a factor ten. This is useful for the production of verythin paints as these have a better adhesiveness and seem to produce less surface disturbances like smallair enclosures. There is also no indication of losses due to re-absorption with high WLS concentrations.For conclusive results higher concentrations need to be evaluated.

At this point Bis-MSB is the best WLS for the WOM detector. Even though Bis-MSB performs wellin general, below 300 nm the capture efficiency drops and does not reach the apparent upper limit of23 %. In order to compensate for this it seems unreasonable to increase the amount of WLS in the paint,as not the WLS capture efficiency but the shifting efficiency seems responsible for the drop as it canbe seen by comparing the absorption (Fig. 5.25) and efficiency (Fig. 5.3) measurement of Bis-MSB.

5 The drop on the very left is caused by the low flux in reference and signal measurement due to absorption in the glass as itbecomes apparent by looking at Fig. 3.6.

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5.1 Development of Wavelength Shifting Paint

Figure 5.4: Efficiencies for different concentration and dipping speeds of Bis-MSB in the same paint mixtureof 400 ml anisole and 85.1 g PMMA. C stands for concentration with H=High= 0.362 g

100ml , M=Medium=

0.126 g100ml and L=Low= 0.065 g

100ml . V stands for velocity with H=High= 9.2(8) cmmin and L=Low= 0.9(3) cm

min .

Therefore it has been thought of improving the performance of the Bis-MSB paint with a secondaryWLS for low wavelengths. For this two candidate WLS have been chosen. First PMP which also dropsat lower wavelength but performs a little better than Bis-MSB. PMP has in addition a very high Stokes-Shift which causes the emission spectrum to be mostly out of the absorption range of Bis-MSB. Thesecond candidate is PT as also described in [48]. Pt has a high photon cross section below 300 nmand its emission spectrum peaks within the efficiency maximum of Bis-MSB. However, there are threedownsides to PT the efficiency could not be measured with a high accuracy due to the high errors in thespectrum measurement, it was measured on borosilicate which reduces the efficiency by a factor of twoto three as it can be seen in Fig. A.5 and PT is hard to solve in sufficient quantities in most solvents.However there are open questions arising from the mixing of two WLS. It is unknown if or how theywill interact chemically. For the mixture PT + Bis-MSB the Bis-MSB emission spectrum can clearlybe assumed for normalization but for the mixture PMP + Bis-MSB the spectrum will change accordingto incoming wavelength and relative concentration of both WLS. Therefore the emission spectrum ofBis-MSB will only be an approximation. It seems intuitive that the WLS with the bluest absorptionspectrum must have a higher cross section at lower wavelength for the light not to be absorbed by thesecond WLS with lower shifting efficiency at those wavelengths. However to accomplish this with thehighest efficiency possible a very accurate fine tuning is necessary, which can not be provided yet as theaccuracy of the efficiency measurement does not allow this and more urging matters like the unexpectedlow efficiency should be addressed first. Therefore only a few crude mixtures are produced and evaluatedyet in order to find a general direction on a two WLS paint. The first results for mixtures of this kindare shown in Fig. 5.5 and the corresponding ingredients are shown in Table 5.3.P41 is just a Bis-MSB paint for comparison. Unfortunately the measurements were conducted with

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5 Development of Wavelength Shifting Wave Guiding Tube

borosilicate6 that due to its high losses does not allow to evaluate the absolute efficiency.P53 shows the attempt to reproduce the WLS concentrations given in [48] with the previously usedanisole and PMMA combination. Thus by trying not to saturating anisole with PT, too few has beenused, causing the dip between 340 nm and 380 . The amount of Bis-MSB used should also be increased.P57 is a mixture taken from [48] and is supposed to be ideal. However only relative measures of theWLS are given without concentrations of the plastic or the solvent. As the efficiency drops below 340 nmthe results are rather disappointing.P56 is also based on the suggested WLS ratio in [48], however due to the high solvability of most WLSin dichloromethane, the PT concentration was chosen to be higher. The results seem very promising.P55 was the attempt of using the mixture from [48] with PMMA as binder in order to later use PMMA assubstrate as well and avoide any differences in refraction index. However, the dissolvability of PMMAin toluene is very limited and will therefore not allow to create paints with higher viscosity that can beused for thicker films. It seemed unreasonable to pursuit this kind of mixture any further.P44 is the attempt to have a paint not based on PT as secondary WLS to compare the other paints to.It shows a very constant efficiency therefore it seemed reasonable to make further tests with this paint.The rather low efficiency might be an effect of the borosilicate and should be evaluated properly on adifferent substrate at a later point.

Figure 5.5: First efficiency measurement for paints with two WLS on borosilicate. Due to the high losses inborosilicate and the use of different plastics and concentrations the total efficiency is here not the indicating factorto determine the best paint. A flat efficiency spectrum is of interest that can later be modified in height by a changeof coating velocity and paint viscosity. The constituents of the paints are listed in Table 5.3.

It was decided to investigate P56 and P44 further. Due to PMP prices and difficulties with dichloro-methane paints on PMMA substrate P57 was also pursued. This seemed a representative choice as three

6 Due to availability at that time.

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5.1 Development of Wavelength Shifting Paint

Name Solvent Plastic Amount WLS 1 Amount WLS 2 AmountP41 Anisole PMMA 23.0 g Bis-MSB 0.36 g - -P53 Anisole PMMA 21.2 g Bis-MSB 0.13 g PT 0.12 gP57 Toluene Paraloid B72 21.3 g Bis-MSB 0.12 g PT 0.25 gP56 Dichloromethane Paraloid B82 32, 7 g Bis-MSB 0.19 g PT 0.35 gP55 Toluene PMMA ≈ 15 g Bis-MSB 0.13 g PT 0.25 gP44 Anisole PMMA 23.0 g Bis-MSB 0.35 g PMP 0.81 g

Table 5.3: Composition of the paints presented in Fig. 5.5. From each solvent 100 ± 5 ml are used. The errors onthe weights are smaller than the shown values. Exception is the PMMA in toluene because it was only partiallysolved and the remainders removed from the solution.

Name Solvent Plastic Amount WLS 1 Amount WLS 2 AmountP64 Toluene Paraloid B72 25.0 g Bis-MSB 0.15 g PT 0.3 gP62 Dichloromethane Paraloid B82 33.0 g Bis-MSB 0.19 g PT 0.36 gP61 Anisole PMMA 29.3 g Bis-MSB 0.44 g PT 0.81 g

Table 5.4: Composition of the paints presented in Fig. 5.6. For each solvent the amounts are scaled to 100± 5 ml.The errors on the weights are smaller than the shown values.

different solvents and plastics are used as well as two different WLS combinations. However the mix-tures were slightly changed in accordance with the previous results. For P44 the amount of binder wasincreased to create thicker films thus improving absorption. The new paint runs under the name P61.P56 was kept the way it is but was produced in higher quantities and runs now under the name P62.For P57 the amount of WLS used were increased and it runs now under the name P64. The mixturesare displayed in Table 5.4 and the corresponding measurements in Fig. 5.6. The best of two to threemeasurements is plotted for each mixture7.P61 shows definitely the worst results and will therefore not be used anymore. P62 in turn looks prom-ising and will certainly still improve if used on glass substrates8 and within a few days after paintproduction to reduce premature solvent evaporation. In comparison it has a very low viscosity and thusproduces very thin films. It shows a very good surface quality on glass as well.P64 shows the best results yet and is best suited for multipurpose use. It still produces cracks in PMMAsubstrate when the exposure time is above a few seconds. But if this can be overcome it might be idealfor PMMA as it does solve PMMA but only in very limited amounts that do allow to coat bigger sampleswithout damaging the substrate thus bonding well with it. A interesting feature is the shift of the P64spectrum to the left in comparison to P61 and P62 although all three paint spectra are expected to bedominated by Bis-MSB. This might be the first hard evidence for binder dependence in WLS behavior.

7 For P64 six previous measurements were conducted by Peter Falke with 23 cm PMMA tubes but all showed just efficienciesof about 16 %. This results could not be reproduced and the reason might be that the tube ends of the short tubes were notpolished like the ones used for the later measurements.

8 The sample showed sever damage to the substrate due to the use of Dichloromethane.

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5 Development of Wavelength Shifting Wave Guiding Tube

Figure 5.6: Analysis of different paints with Bis-MSB and a second WLS to improve performance in the 280 nmto 330 nm regime. All measurements were taken on 30 cm PMMA tubes with the lower 5 cm coated with paint.The paint composition is given in Table 5.4. The P62 paint was already visibly aged due to solvent evaporationand plastic deposits at the container walls. In contrast to previous results with the paint the surface quality sufferedvisibly. This is because the dichloromethane in the paint solved part of the tube surface..

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5.2 Paint Substrates - Wave Guides

5.2 Paint Substrates - Wave Guides

For the tube shaped wave guide which is supposed to be coated with the paint, three materials havecurrently been taken into consideration: Borosilicate glass, Quartz glass and PMMA. To understandtheir suitability for the use as wave guide each of them has been coated with a anisole, PMMA andBis-MSB paint at the same velocity9, because this paint is currently best understood. Afterwards thecombined shifting and wave guiding efficiency was measured with the setup described in section 3.4.The results are presented in Fig. 5.7. It can be seen that PMMA and quartz glass perform about equallywell. However borosilicate glass performs considerably worse. One reason for that is suspected to bethe in-transparency for UV-light of borosilicate. If not all of the light is absorbed and re-emitted by theouter layer of the paint the remaining UV-light can in borosilicate not pass through the tube. This wayit is absorbed by the tube before reaching the second layer of WLS paint on the inside. UV-transparentmaterials like quartz glass and PMMA do not have this disadvantage. For completeness it should bementioned that the efficiency measured for PMMA and quartz glass may also fluctuate. For instance inboth cases efficiencies of just ≈ 17 % could also be observed. Thus in both cases the surface of the paintshowed irregularities, which can not always be avoided. Therefore no conclusions for the performancebetween quartz glass and PMMA can be made from the differences to be seen in Fig. 5.7, even thoughthe statistical error is smaller than the differences.

Figure 5.7: Efficiency measurement of the same PMMA, anisole and Bis-MSB paint coated with the same velocityon the three substrates PMMA, borosilicate glass and quartz glass.

However measuring the losses per distance between the light entering the tube and the PD with thesetup described in section 3.5 for each of the three materials as seen in Fig. 5.8, shows that these losses

9 Same film thicknesses are assumed, but especially for PMMA this can not be guaranteed. Because part of the PMMA tubessurface is always solved.

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5 Development of Wavelength Shifting Wave Guiding Tube

are also significantly higher for borosilicate glass. This can not be explained with UV-light absorptionas the light emitted by the WLS is blue.Quartz glass performs not as good as PMMA. This tendency has been observed in general, however onoccasions results as good as with PMMA could be observed with quartz glass. Therefore it is assumedthat the tube quality does fluctuate and has detectable impact on the efficiency. This can be seen in thefunction shapes of PMMA and quartz glass as well, because fluctuations bigger than the measurementerror occur. These may be caused by varying in-perfections of the substrate or the surface of the paint.

Figure 5.8: Efficiency behaviour as function of liquid crystal light guide position relative to the PD. Used are thethree different substrates with the same anisole, Bis-MSb, PMP and PMMA paint.

In order to investigate the losses further photographs according to section 3.7 have been taken (Fig.5.9, 5.10 and 5.11). On the first attept to coat paint on borosilicate the paint was repelled and it turnedout that residues10 on the surface of the tube were responsible for that. Therefore each substrate isphotographed once before the cleaning and once afterwards. However it could not be ensured, thatalways the same part of the tube faces the camera. Photos were taken with a sensitivity correspondingto ISO 3200 with 15 s exposure time with a Canon EOS 1100D and a 18 − 55 mm objective. All tubeshave a diameter of 2 cm, thus the tubes wall thickness is different. In fig. 5.9 the PMMA sample can beseen. with a wall thickness of 2 mm, which means that more light can couple in than in any of the othertube types. The untreated tube shows a milky film which is probably caused by oil residue from thecutting and polishing machinery. Also small spots can be seen, it is unclear whether they are superficialmarks from the production or air enclosures within the material. However it should be mentioned thatthese tubes are not made for optical purposes and they show rather a polygonal shape than a roundshape when looking at them under an angle. To clean the tube a plastic cleaner from evonik [50] hasbeen used, because solvents seem to attack the surface to strongly and / or created tension cracks in the

10 probably oil

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5.2 Paint Substrates - Wave Guides

material. Afterwards the tube was rinsed with distilled water and blown dry with pressurized air. Due tothe cleaning the milky film disappeared. However, small scratches are visible that were not there before.They were probably caused by the cleaning, though they may also have been invisible before due to themilky film.

Figure 5.9: PMMA substrate illuminated by an integrating sphere and photographed. Untreated tube on the topand cleaned tube on the bottom.

In Fig. 5.10 a quartz glass sample is shown, with a wall thickness of 1.3 mm. The uncleaned sampleshows residues that are probably caused by the glass cutting. Both the untreated and the cleaned sampleshow small spots. Similarly to the PMMA sample, it is unclear whether they are superficial or not. Thecleaning was done with water, dish wash liquid, aceton and isopropyl alcohol. Yet the cleaned sampleshows a milky structure that was not there before. It might be residue from the dish wash liquid, howeveradditional cleaning with the two solvents did only worsen it. The same cleaning combination has beenused on the borosilicate tube and it did not show these structures. Therefore it might be the interactionof the glass with the cleaning liquids and additional cleaning methods should be considered.

Figure 5.10: Quartz glass substrate illuminated by an integrating sphere and photograped. Untreated tube on thetop and cleaned tube on the bottom.

In Fig. 5.11 a borosilicate sample is shown, it has a wall thickness of just 1 mm and therefore allowsfor the least light to enter the tube. The untreated sample is covert with some residue from the productionprocess which is responsible for repelling the paints when not cleaned. The cleaned and non-cleanedsamples display faint spots as with the other tube types. The overall brightness is much lower, which isparcially caused by the smaller wall thickness. However it might also be an indication that the stronglosses in borosilicate as seen in Fig. 5.8 are due to absorption11.

The last thing that should be taken into account is the general compatibility of the paints with thesubstrate. Borosilicate and quartz glass are not damaged by any paint and if cleaned properly all paintsform a nice film. However after several days to weeks the inside coating of the pips peels off even with11 Unfortunately it was not possible yet, to accurately determine the absorption of tubes independently of any paint.

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5 Development of Wavelength Shifting Wave Guiding Tube

Figure 5.11: Borosilicate glass substrate illuminated by an integrating sphere and photograped. Untreated tube onthe top and cleaned tube on the bottom.

the custom paints, while the outside coat on the tube stays intact. This probably happens because thepaint shrinks when it dries and also due to different expansion coefficients12.With PMMA in turn this is not an issue. However aggressive solvents like dichloromethane solve greatamounts of plastic from the surface of the tube within minutes, which changes paint concentrationand causes bad paint surfaces after the coating. With milder solvents like anisole and toluene thisis an advantage. as only small amounts of the surface are solved thus binding paint and substrate welltogether. This even allows paint and substrate to be from the same material thus avoiding any differencesin refraction index. On the contrary, on several occasions cracks in the substrate have been observed afterexposure to paint or any other kind of solvent (Fig 5.12). It was supposed that these cracks are caused bytension created when the plastic was cooled down too rapidly in the production process. Therefore it isplaned to test the behavior of a tube after tempering13 it. It was suggested by Zander Moss14 to heat upthe tubes before the coating as they encountered similar problems with smaller cracks. First tests werepromising concerning the cracks, however it was difficult to keep the tube in shape during the coatingprocess when it is heated up.

Figure 5.12: Cracks in PMMA after contact with acetone for about a minute. Residue of the solvent and plasticwas whipped of the tube before the photograph was taken. Acetone attacks PMMA faster than most solvents do,however even with toluene these cracks could be observed within minutes. Considering a 1 m tube and the normalcoating velocity, exposure times are longer than that.

In the end the measured results with PMMA look most promising, combined with the fact that thePMMA tubes used until now are of rather low quality, The good adhesiveness and the promising resultson solving the problems with the cracks PMMA is suggested as a substrate for the next research steps.However this suggestion does not rule out other materials or totally different approaches that mightreduce losses better.

12 The coating for the tube used in the WOM will not be coated on the inside when this does not decrease the efficiency.13 Slowly heating up the substrate followed by a slow decrease in temperature back to room temperature14 Massachusetts Institute of Technology

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5.3 Losses

5.3 Losses

As previously already mentioned several times, the highest achieved efficiency with various paints isabout 22 ± 2 % but the highest expected efficiency is 74.6 %. in this section the source of these lossesand possible counter measures are examined.The first indication for the losses is the error on the measured efficiency of 22 ± 2 %. The result oftwo measurements of the most efficient paint available on the same tube with the same PD and liquidlight guide position are shown in Fig 5.13. The only difference between these two measurements is arotation of the tube, around its symmetry axis in the measurement setup. The results can by reproducedwithin the error when the tube is turned back, therefore it is certainly caused by the sample and not theSetup. In combination with the fluctuating and not only decreasing efficiency in Fig. 5.8, this is a strongindication for losses due to variable substrate and or paint quality. However with the measurementstaken until now it is not possible to determine which of this two factors is dominant. Thus Fig. 5.9,5.10 and 5.11 illustrate that there are losses due to the substrate. It is just unclear whether they have asignificant effect as the pictures do not allow an absolute measurement yet.

Figure 5.13: The same efficiency measurement with the PMMA tube turned along the symmetry axis in between.

Exemplary Fig. 5.14 demonstrates that the paint changes the way light is scattered from the tube.The scattered light in the coated region of the tube seems brighter and more homogeneous. Howeverthis part is closer to the source that emits light at 360 nm and can not be seen directly with the camera.While the coated paint is emitting light where excited. Measurements to quantify this behavior are notavailable yet, but it is recommended to analyse this behavior for fully understand the current efficiencymeasurements.

Furthermore the quantum efficiency (QE) of the used WLS is determined to ensure a value close to100 %. In order to obtain the QE the measurement method described in section 3.6 was used and theresult is shown in Fig. 5.15. This measurement has just been done once yet for a Bis-MSB, anisole and

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5 Development of Wavelength Shifting Wave Guiding Tube

Figure 5.14: Illuminated PMMA tube coated with P61 on the far end for about 5 cm. A difference ins lightscattering behavior can be observed.

PMMA paint sample, meaning that the experience with this measurement is still limited. There is anabsorption effect still unaccounted for. It is unclear if all of the light is absorbed by the front layer ofpaint on the microscope slide, because the glass used for the microscope slide shows absorption in Fig.3.6. If existing, the part of the light that is not absorbed by the WLS in the first layer is unknown andcan not be accounted for. However, it can be worked around that by using a extreme thick layer of paint,which will be tried in the near future.The maximum efficiency measured while including the tube was about 22 ± 2 % which is about thesame as the peak efficiency from the commercial paint, the quantum efficiency of the commercial paintis stated to be ≥ 90 % [38]. Thus making both results consistent with one another. The rather largeerror is caused by the large amount of measurements necessary for this result as well as due to the lowlight intensity caused by the long light path, isotropic emission/reflection of the light and the UV lightabsorbing optical devices used. The low intensity of the light is also the reason for only producingreasonable results until 320 nm15.

It also has been looked for losses due to re-absorption. The emission spectrum of the WLS was takenfrom a tube end while illuminating the paint once at the lower end of the tube and once just at the upperend of the coated area on the tube. The result is shown in Fig. 5.16. On the left side of the spectrum asmall excess can be seen that exceeds the standard error. But this difference is marginal and it can notbe verified, due to the error, whether the re-absorbed light is re-emitted again at another wavelength.Measurements at different positions between the shown ones have been taken, too. However in compar-ison to each other they did not show significant differences considering their error. Meaning the processis rather a continues change.

Based on the results from Fig. 5.15, it is possible to reconstruct the expected efficiencies for themeasurement described in section 3.4, according to the known losses. The first losses that are notaccounted for yet are the fresnel losses due to reflection of the incoming light. According to eq. (2.9)the losses are 4.0 % for light perpendicular to the surface of a material with refraction index 1.5. Thecapture efficiency of 74.6 % has to be accounted for. Also the PD angular efficiency. This is done withthe simulation described in Fig. 3.10 and the help of the results shown in Fig. A.6. The resultingfunctions are displayed in Fig. 3.11. For large distances x from the PD the PD capture efficiencyresults in 67.3 %16 17. The expected efficiency in Fig. 5.17 is obtained from the product of these three18

15 Below 320 nm the amount of detected light decreases to virtually zero. Further modifications of the setup in the future willimprove on that.

16 By using this result, losses due to absorption or scattering that would not have reached the PD anyway are hidden. Especiallybecause huge angles mean long light path and a higher probability for absorption or scattering.

17 This accounts only for the one dimensional acceptance. However the second dimension is limited to the high acceptanceregime due to the total internal reflection angle and its contribution are therefore neglectable.

18 For all calculations in this thesis it is assumed that all used plastics and glasses have e refractive index of 1.5. However this

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5.3 Losses

Figure 5.15: The efficiency of the WLS is shown normalized to spectralon and corrected for lens absorption. Thelarge error is caused by low light intensity and the large amount of measurements necessary to obtain this result.

Figure 5.16: Here is the spectrum taken at different positions of a coated tube shown. You cans see the differencein shape at about 410 nm. This is the part where re-absorption takes place.

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5 Development of Wavelength Shifting Wave Guiding Tube

factors and the results from Fig. 5.15. Also shown is the measured efficiency on a PMMA tube19.The discrepancy in the results on the right is therefore most probability caused by absorption and/orscattering of the light in the tube or the paint.

Figure 5.17: Comparison of the expected efficiency based on Fig. 5.15 and known losses, to the measured effi-ciency.

Ifn the substrate and paint surface quality can be enhanced for optimum wave guiding with minimumabsorption, about a factor of two in efficiency can be gained. Thus making the investigation of thesequantities a primary subject for the near future.

is an approximation and may lead to additional losses.19 All results are obtained from the same PMMA, Bis-MSB and anisole paint.

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CHAPTER 6

Full Module Efficiency

To get an estimate on the actual performance of the proposed detector module it is of uttermost interestto investigate its projected efficiency. For comparison, the corresponding efficiency of the DOM will beused. For this calculations the results for the efficiency of P64 are used. However these results couldnot be reproduced in all tube positions, which means that for a detector with this efficiency the lossesmust be further investigated and better understood in order to avoid them. But as some parts of the tubedo not show these losses it is assumed that with some effort at least the maximal measured efficiencycan be achieved, justifying that the best result yet is taken. Also, as displayed in Fig. 5.8 the efficiencydepends on the position of the incoming photon relative to the detector. The current efficiency is for atransmission length of ≈ 26 cm in a 30 cm PMMA tube. The final detector is planned to have a tube of1 m length and 9 cm diameter, that is read out at both ends. Therefore the following calculations shouldbe considered an approximation.The basic efficiency of both the WOM and the DOM is shown in Fig. 6.1. The DOM efficiency isobtained from a measurement under the angle of maximum efficiency (cos(α) = 1) [23]. The WOMefficiency εWOM(λ) is determined by the product of paint efficiency εWLS(λ) and the convolution of PMTefficiency εPMT(λ) and the to one normalized WLS emission spectrum S WLS(λ) as shown in eq. (6.1).

εWOM(λ) = εWLS(λ) ·∫ ∞

0S WLS(λ) εPMT(λ) dλ (6.1)

In order to choose a PMT, the QE of the most suited models has been obtained as imaged in Fig.A.7. The best suited PMT in with respect to the QE for the WLS emission spectrum is the GaAsP PMT,however it has a noise rate of currently ≈ 100 Hz [51], which is much higher than the required 10 Hz.In comparison, the other PMTs with ≈ 10 Hz [27] meet the requirements. Also the GaAsP PMT iscurrently only available with a photosensitive area of 18 mm diameter [52], as it is difficult to producethis kind of PMT in bigger sizes. This would in turn make it necessary to make the walls of the innercylinder as thin as 0.45 mm. Due to those reasons the GaAsP PMT is not further considered at themoment, although it would increase the overall efficiency by 35.4 %. The next best PMT is the EG PMTwhich is used for the calculations in Fig. 6.1. From that results the WOM would perform about a factortwo worse in efficiency as the DOM, however there are factors not yet accounted for.

The angular acceptance for both detectors is different and must be taken into account. The angularacceptance for the DOM has been measured in [23]1. The angular acceptance for the WOM has not

1 It is unclear how the measurement is conducted. When only a small spot of the detector was illuminated under each angle

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6 Full Module Efficiency

Figure 6.1: Basic efficiency of WOM and DOM. For the WOM this includes the product of paint efficiency andthe convolution of PMT efficiency with the paint emission spectrum. For the DOM the efficiency for light comingunder α = 0.

been measured yet and is therefore calculated from the known basic geometry and refraction indeses.For the geometric considerations it is assumed that the area spanned by the angle θ is perpendicular tothe symmetry axis of the detector. The area spanned by α is parallel to the symmetry axis of the detector.In order to enter the detector the light passes from ice with a refraction index of ≈ 1.33 to quartz glasswith a refraction index of ≈ 1.5. Afterwards the lights needs to pass from the quartz glass into air ornitrogen with a refraction index of ≈ 1. With the approximation that the radius of the outer quartz glasscasing is much larger than the wall thickness it can be assumed that both surfaces are parallel. Now thetransmittance and therefore angular acceptance can be calculated using the Fresnel eq. (2.9) and Snellslaw eq. (2.5), resulting in eq. (6.2). Eq. (6.3) gives the angle between the incoming light and the surfaceof the detector. Eq. (6.4) does the same but is corrected for the directional change of the light path whenentering the glass according to Snells law. As the detector is cylindrically symmetric, it can be integratedover θ, as shown in eq. (6.5). An additional factor of sinα is introduced to take projection effects intoaccount. The result of this calculation and the measurement results from the DOM are shown in Fig.6.2. By integrating over all angles the overall angular efficiency of the DOM is 34.1 % and of the WOM57.5 %.

the results would only be one dimensional. When the hole detector has been illuminated with a parallel beam the data wouldinclude two dimensions. The later one is considered for the further calculations as this would be from a physics point ofview the most reasonable measurement.

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εangular(α, θ) = R(nice, nglass, β(α, θ)) · R(nglass, nair, β′(α, θ, nice, nglass)) (6.2)

β(α, θ) = sinα cos θ (6.3)

β′(α, θ, n1, n2) = arcsin(n2

n1sin β(α, θ)

)(6.4)

ε′angular(α) = sinα∫ π

2

− π2

εangular(α, θ) cos θdθ (6.5)

Figure 6.2: Angular acceptance for the WOM and the DOM, calculated according to eq. (6.5) and measured [23],respectively.

Including the texturing of the casing from the inside as described in section 2.2, an additional gainin efficiency of 37 % is possible according to [53]2. Also the fact that the Cherenkov spectrum isproportional to ∝ 1

λ2 as shown in section 2.1.1 needs to be taken into account. And last the detectioncross section of the detectors needs to be taken in to account. For the WOM this is expressed by eq.(6.6) with the length L = 1 m of the detector and RWOM = 0.1 m the detector radius, eq. (6.7) gives thesame for the spherical DOM with radius RDOM = 0.15 m.

AWOM = 2RWOML (6.6)

ADOM = πR2DOM (6.7)

2 This number is obtained according to numerical simulations with optimal shaping for the lenticular array. In praxis meas-urements, a lower gain could be measured. However this is most likely due to the unknown shape of the arrays and willimprove when the array shape matches the optimum shape as calculated by Peter Falke.

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6 Full Module Efficiency

Taking all these factors into account results in Fig. 6.3. As clearly visible the WOM is superior tothe DOM and profits a lot from the increased amount of photons for lower wavelength. In numbers,this means the efficiency of the WOM is higher by a factor of 2.7. When including wavelengths below280 nm the factor is likely to increase.

Figure 6.3: WOM and DOM efficiency weighted by the Cherenkov spectrum, detector area, angular acceptanceand in case of the WOM transmission improvements due to surface texturing.

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CHAPTER 7

Conclusion

7.1 Results

During the work on this thesis, a dip coater was upgraded and put into operation. The coater andthickness of the paint films produced with it have been investigated thoroughly. The results were wellreproducible and suitable for our purposes. Several optical setups have been used and developed inorder to evaluate the optical properties of wavelength shifting paints. This way a good insight intothe properties and possible improvements of WLS paints could be obtained. With that it was possibleto fabricate three highly efficient WLS paints. The three paints are based on different binders andsolvents. Currently the best result is obtained with 25.0 g Paraloid B72, 0.15 g Bis-MSB and 0.3 g PTper 100 ml Toluene. With the used measurement setup, good performance in the range between 280 nmand 400 nm could be verified. In the range of 250 nm to 280 nm, the input light intensities have beentoo low to produce accurate results, but a non zero efficiency can be expected. However when coatedon plastic or glass tubes, only efficiencies of 22 ± 2 % could be achieved whilst 74.6 % is expected inthe ideal case. About half of the losses could be accounted for. The remaining losses are expected tocome from scattering and absorption in the light guiding process. Where exactly and why this happensremains unknown. Moreover three different substrates could be evaluated and PMMA seems currentlythe most promising candidate in therms of efficiency for further developments followed closely by quartzglass. Applying the developed WLS paint albeit including the large losses, the efficiency of the newsingle photon detection unit, the wavelength shifting optical module (WOM), could be estimated to bea factor 2.7 higher than the efficiency of its predecessor, the digital optical module (DOM), as used inthe IceCube neutrino detector, whilst reducing the noise rate by a factor of ≈ 50.

7.2 Outlook

In order to exclude errors on the PD measurements, the next step should be to investigate the sameparameters with a PMT setup, where accurate singe photon counting is possible. Also tests on largerhigh quality PMMA tubes are a logical continuation of the work. If the measured losses are reproduced,effort should be made to reduce the avoidable losses by improving substrate and paint surface quality.Also testing the WLS composition determined in this thesis in different setups could be an option.For instance an optical fiber setup or WLS in a liquid solution, contained in a glass housing, can bea possible detector approach. The latter could be interesting in the light of combining research effort

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7 Conclusion

with the development team of the ZOMBI1 detector at DESY Zeuten. If the losses are not reproducedor significantly lower the next step would be the assembly of a first prototype and its evaluation. Inaddition once the varying losses have been eliminated, it should be possible to fine tune the weight ratiobetween the two WLSs and the binder in order to maximize efficiency further.

1 A wavelength shifting detector based on similar light guiding technology whilst using a liquid container for the WLS andno casing for the inner detector unit.

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APPENDIX A

Useful information

A.1 Dye theory

σ-bonds are associated with wavelengths in the UV. σ-bonds are the strongest type of covalent bondsand are created when two orbitals overlap head on. π-bonds in turn are associated with excitationenergies in the optical regime. π bonds are also a covalent bond and are created when two paralleloriented p-orbitals are connected in a way that each lobe of the p-orbital meets the corresponding oneof the other atom. If several bounds of this kind are combined the π electrons become indistinguishablefrom each other, in other words they become delocalized. This allows to move absorption and emissionspectra towards longer wavelengths. A C=C-bond consists of a σ-bond and a π-bond, it is ideal forwavelength shifting purposes. This also explains why most WLS have C=C-bonds,in most cases evenone or more benzene rings (which has three such C=C-bonds) like shown in Fig. A.1.

Figure A.1: Benzene ring and its bonding structure on the top and left (modified version of [54]). Chemicalstructure of Bis-MSB (1,4-Bis-(2-methylsteryl)benzene), a WLS, on the right, showing the importance of C=Cbounds for the wavelength shifting process.

A.2 Additional Material

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A Useful information

Figure A.2: Predicted interaction lengths for neutrino interactions on nucleon targets: dotted line, charged-currentinteraction length; dashed line, neutral-current interaction length; solid line, total interaction length. The dot-dashed curve shows the charged-current interaction length based on the EHLQ structure functions. Lmathrmint isgiven in the water-equivalent interaction length per cm. [6]

Delay time in min velocity in cm/min velocity error in cm/min1 3.21 18.536 0.3272 7.36 8.084 0.1373 10.55 5.640 0.0955 16.92 3.517 0.059

10 32.09 1.854 0.03120 64.19 0.927 0.01640 128.30 0.464 0.008

Table A.1: Velocity calibration of the dip coater. All measurements are done for a distance of 59.5 ± 0.2 cm. Thetime has been stopped by hand and is therefore estimated to be ±1s. Compared to the smallest measured time thisis about 0.4 %.

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A.2 Additional Material

Figure A.3: Transmittance of quartz glass and borosilicate glass in comparison. Both are corrected for Fresnellosses (section 2.2.3). Data was taken by [55] on glass samples intended as casing for the WOM. Unfortunatelyno errors are available. However size of entries grater than one might be an indication for the error size. The nonphysical values (> 1) might also be caused by the focusing effect of a denser medium.

Figure A.4: Relative transmission spectrum of a lens with focal length f = 50 mm.

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A Useful information

Figure A.5: Efficiency measurement of PT in PMMA / anisole paint as used in Fig. 5.2 and correspondingspectrum. However in comparison to the plots in Fig. 5.3 a borosilicate tube with 20 mm diameter and 1.0(5) mmwall thickness was used. As tested in different measurements the efficiency on borosilicate tubes is about a factortwo to three lower than on PMMA or quartz glass.

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A.2 Additional Material

Figure A.6: Angular capture efficiency of the PD used in all measurements conducted in this thesis. Obtainedby shining approximately parallel light on to the PD while turning it on the spot. Afterwards it was correctedfor projection effects. Unfortunately the DAQ for angular measurements does not support the determination of astatistical error.

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A Useful information

Figure A.7: Efficiency for three potential PMT’s to be used in the WOM. UBA and EG are ultra bialkali andenhanced green ultra bialkali PMT’s respectively from Hamamatsu [27]. GaAsP corresponds to the R9792U-40MHP0119, also from Hamamatsu. [52]

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[41] P. Lin et al., “Dip Coating Assisted Polylactic Acid Deposition on Steel Surface: Film ThicknessAffected by Drag Force and Gravity”, English,13th International Conference on Biomedical Engineering, ed. by C. Lim and J. Goh, vol. 23,IFMBE Proceedings, Springer Berlin Heidelberg, 2009 1341–1343, isbn: 978-3-540-92840-9,doi: 10.1007/978-3-540-92841-6_330,url: http://dx.doi.org/10.1007/978-3-540-92841-6_330 (visited on 24/08/2014).

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List of Figures

2.1 Neutrino cross section across energy scales [6]. As concrete example only the elec-troweak cross-section νee− → νee− is shown. The peak at 1016 eV is caused by theGlashow resonance [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Feynman diagrams showing the reaction of a neutrino with the particles in the ice.Charged current (CC) interactions are shown on the left and neutral current (NC) in-teraction on the right.The top row holds the graph of hadronic interactions of neutrinoswith a quark of a nucleus. On the bottom interactions of neutrinos with shell electronsare depicted. Time goes from bottom to top. . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Diagram of a Cherenkov light cone. In red, the direction of the particle traveling withvelocity βc > c

n the phase velocity of light in the traversed medium. In green, theresulting Cherenkov light wave front is shown. . . . . . . . . . . . . . . . . . . . . . 6

2.4 Sketch of the IceCube detector setup [21]. The main IceCube detector including theDeepCore sub-array consists of 86 sensor strings with 60 sensors each. On the surfaceis the air shower/veto detector IceTop consisting of 81 stations with two ice tanks andtwo PMTs (Photo Multiplier Tubes) each. The IceCube predecessor, AMANDA II isshown within the IceCube detector volume. . . . . . . . . . . . . . . . . . . . . . . . 7

2.5 An event observed in August 2011 [24], the so called “Bert”-event named after thecharacter in Sesame Street. Each sphere represents a DOM in the detector. The colorsrepresent the arrival times of the photons with red indicating early and blue late times.The size of the spheres is a measure for the recorded number of photo-electrons. Thereconstructed energy is 1.04 ± 0.16 PeV. [25] . . . . . . . . . . . . . . . . . . . . . . 8

2.6 Schematic of an IceCube DOM (Digital Optical Module) [21]. Showing the glass hous-ing surrounding in the lower part a PMT shielded by the mu-metal grid embedded inRTV gel and the electronics in the upper part. The electronics splits up in the HV di-vider at the top, LED flasher board in the upper center and the dalay as well as the mainboard in the center. On the upper half the penetrator is fixed to allow for power inputand signal transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.7 Schematic of the Wavelength shifting Optical Module (WOM) on the right [27]. Theworking principle of a wavelength shifting wave guide is shown on the left. Light isabsorbed by a thin layer of wavelength shifter (WLS) and re-emitted isotropically at alower wavelength. With a certain probability the re-emitted light is captured by totalinternal reflection and guided towards the PMT. . . . . . . . . . . . . . . . . . . . . . 10

2.8 Visualization of the state changes necessary for wavelength shifting, explaining the dif-ference in absorption and emission wavelength. [29] . . . . . . . . . . . . . . . . . . 12

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List of Figures

2.9 Illustration of Stokes-Shift. In blue the Absorption spectrum, yellow the emission spec-trum and green the overlap of both which leads to re-absorption, ergo in-transparencyand losses in efficiency for a WLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.10 Visualisation of the Fresnel equations (transmittance/reflection and their angular de-pendence). On the left side the reflection coefficient for randomly polarized light com-ing from a refraction index n = 1 and going to a refraction index n = 1.5. On the rightside the same for light going from n = 1.5 to n = 1. Important is the total internalrefraction range. All the light within that angular range is captured inside the mediumunless the angle of incidence changes. . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.11 Illustration of total internal reflection in a cylinder cross-section (left) and illustration oflight capturing in a glass plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.12 Glass surface texture for improved transmittance from ice to glass to air with lenticulararrays (by Peter Falke). On the left without texturing, in the center with a triangulartexturing and on the right with circular texturing. . . . . . . . . . . . . . . . . . . . . 16

3.1 The dip coater, (Left) Before the wrapping with colored marks for explanation. The stepmotor (blue) can be seen, the casing for the turning rod (yellow), the carriage (green),the moving range limiting switches (red and green), control unit (purple) and the paintvessel holder (black). (Right) After the coater has been wrapped and equipped with theexhaust at the top left corner, for extraction of paint vapurs. . . . . . . . . . . . . . . . 18

3.2 Determination of dipping velocity depending on the internal delay in the dipcoater con-trol. A fit resulted in an inverse slope of m = 18.558 ± 0.009 min

cm·ms . . . . . . . . . . . . 193.3 Schematic setup of the absorption measurement. The arrows indicate the light path onto

the photodiodes (PDs) where the light is captured. . . . . . . . . . . . . . . . . . . . . 203.4 Light source and reference setup. The violet arrows indicate the light path. The light

from the monochromator slit is projected onto the liquid crystal light guide. For refer-ence a beam splitter In the beam path takes light out of the beam. The light used forreference is reduced by means of a collimator. . . . . . . . . . . . . . . . . . . . . . . 21

3.5 Picture of a microscope slide in the absorption setup . . . . . . . . . . . . . . . . . . . 223.6 Absorption measurement with a microscope slide normalized to a measurement without

slide. Below 280 nm the absorption of the glass is too high for transmission measure-ments, although it has to be taken into account that measuring without a slide changesthe geometry of the setup. Thus Fresnel losses are not canceled by normalization in thatparticular measurement. A measurement with Poly(methyl methacrylate) (PMMA) filmon a microscope slide normalized to a microscope slide is shown as well. It can be seenthat a thin PMMA film without WLS does not increase the absorption. . . . . . . . . . 23

3.7 Schematic setup of the efficiency measurement. The arrows indicate the direction andcolor change of the light. Within the tube wall the light propagates in a spiral fashion. . 24

3.8 Top view on the measurment setup with a PMMA tube sample coated with a PMMAbased Bis-MSB (WLS) paint at the right side. The paint is illuminated by the liquidcrystal light guide with 360 nm light and read out with a photodiode at the left end. Thetotal length of the tube is 28 cm. On the right side is a black plastic plate connectedto the tube by optical gel in order to absorb the light at that end. However it is clearlyvisible that not all the light is absorbed. . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.9 Schematic setup for a measurement to determine losses with positional dependence. . . 273.10 Schematic sketch for the working principle of the geometric transmission calculation. . 28

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List of Figures

3.11 Results of the geometric dependence calculations for d = 2π cm and w = 0.6 cm. Thefirst two calculations (black, blue) are just based on plain geometry. The second two(red, yellow) also include the angular acceptance of the PD (see Fig. A.6). Based onthis calculation it is also possible to simulate different absorption lengths and comparethose to the measured ones. However a qualitative analysis of this has not been done yet. 28

3.12 Schematic setup of the WLS quantum efficiency measurement. Light is projected ontoa WLS paint coated microscope slide and the emission is measured. For normalization,the reflected light from the Lambertian reflector spectralon is used. For confirmationthat the WLS emits Lambertian too, the arm with Signal PD and corresponding lens canbe rotated by the angle β. The light source and reference setup is the same as before. . 29

3.13 The intensity distribution obained in the angular measurement of spectralon, a WLSpaint coated microscope slide and a plain microscope slide. The proportions betweenthe measurements are as shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.14 Scaled data from Fig. 3.13 with the attempt to correct for the reflection peak of theWLS paint coated slide using the plain measurement of a microscope slide. At anglesclose to −90 the intensity of the from the WLS emitted light is very low. Thus it can beassumed that the error has a size comparable to the difference to the spectralon intensity.Therefor, it is assumed that both functions match. . . . . . . . . . . . . . . . . . . . . 32

4.1 Reproducability of paint layer thickness as a function of the dip coater velocity and thean empirical fit to this correlation. It should be pointed out that these values are averagedover the paint surface and do not assure even paint distribution. . . . . . . . . . . . . . 34

4.2 Absorption measurements for different paint thicknesses. The black dashed lines are asecond line of samples with the same coating velocity to test reproducability of thesemeasurement technique. Except for the 0.02 mm measurement, all fit well within theirerror. The reason for the difference with the 0.02 mm sample is unclear. However dueto the very small discrepancy it was not given much attention and might just be causedby a slightly tilted slide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.3 Exemple fit for the mean free path (eq. (4.2)) at 375 nm with the values obtained fromFig. 4.2. Producing the following results: a = 0.42.08 µm, λ = 8.33.08 µm andχ2/d.o.f= 26.7. The rather large χ2/d.o.f is probably du to the not included error ofthe thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4 Collection of all fitted mean free path results as shown in Fig. 4.3. For comparison,0.02 mm is about the maximum reachable paint thickness. . . . . . . . . . . . . . . . . 36

4.5 Efficiency of commercial paint and the corresponding emission spectrum, measured ona 30 cm PMMA tube with 2 cm diameter and 2 mm wall thickness. The paint layer is0.02 mm thick, according to the measurements in Fig. 4.1. The slight variation betweenthe shapes from the absorption spectra in Fig. 4.2 is probably caused by the change ofrefraction index with wavelength of the PMMA. . . . . . . . . . . . . . . . . . . . . 37

4.6 Absorption spectrum of the commercial paint with additional P-therphenyl (PT) solvedin maximum solution and the paint without additives for comparison. (Due to fluctu-ations the value at 305 nm has been removed.) It is clear that some improvements canbe made this way, but due to limited dissolvability the improvements are limited aswell. The large errors compared to Fig. 4.2 are caused by a lower light intensity in thismeasurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.7 A Picture showing the peeling of the paint on a microscope slide (left) and a opaquepaint layer on quartz glass tube (right) . . . . . . . . . . . . . . . . . . . . . . . . . . 40

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List of Figures

5.1 Absorption of a PMMA paint with and without PT. Due to the low solvability of PT inanisole not all of the light can be absorbed. . . . . . . . . . . . . . . . . . . . . . . . 42

5.2 Absorption of different WLS in the same PMMA / anisole paint in absolute values (inblue on the left). And the corresponding relative emission spectra (in red on the right).For some WLS the error is bigger than the measured values due to the high signalto noise ratio of the spectrometer for low intensities (section 3.3). It might be a firstindication for a low wavelength shifting efficiency. . . . . . . . . . . . . . . . . . . . 45

5.3 Efficiency of different WLS in the same solvent/binder combination and the correspond-ing relative emission spectra. The errors on Butyl-PBD are extremely high due to thefolding with the emission spectrum which is very poorly known. PT was not measuredhere due to a lack of PMMA tubes, but at a later point (see Fig. A.5). . . . . . . . . . . 46

5.4 Efficiencies for different concentration and dipping speeds of Bis-MSB in the same paintmixture of 400 ml anisole and 85.1 g PMMA. C stands for concentration with H=High=

0.362 g100ml , M=Medium= 0.126 g

100ml and L=Low= 0.065 g100ml . V stands for velocity

with H=High= 9.2(8) cmmin and L=Low= 0.9(3) cm

min . . . . . . . . . . . . . . . . . . . . 475.5 First efficiency measurement for paints with two WLS on borosilicate. Due to the high

losses in borosilicate and the use of different plastics and concentrations the total ef-ficiency is here not the indicating factor to determine the best paint. A flat efficiencyspectrum is of interest that can later be modified in height by a change of coating velo-city and paint viscosity. The constituents of the paints are listed in Table 5.3. . . . . . . 48

5.6 Analysis of different paints with Bis-MSB and a second WLS to improve performancein the 280 nm to 330 nm regime. All measurements were taken on 30 cm PMMA tubeswith the lower 5 cm coated with paint. The paint composition is given in Table 5.4. TheP62 paint was already visibly aged due to solvent evaporation and plastic deposits at thecontainer walls. In contrast to previous results with the paint the surface quality sufferedvisibly. This is because the dichloromethane in the paint solved part of the tube surface.. 50

5.7 Efficiency measurement of the same PMMA, anisole and Bis-MSB paint coated withthe same velocity on the three substrates PMMA, borosilicate glass and quartz glass. . 51

5.8 Efficiency behaviour as function of liquid crystal light guide position relative to the PD.Used are the three different substrates with the same anisole, Bis-MSb, PMP and PMMApaint. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.9 PMMA substrate illuminated by an integrating sphere and photographed. Untreatedtube on the top and cleaned tube on the bottom. . . . . . . . . . . . . . . . . . . . . . 53

5.10 Quartz glass substrate illuminated by an integrating sphere and photograped. Untreatedtube on the top and cleaned tube on the bottom. . . . . . . . . . . . . . . . . . . . . . 53

5.11 Borosilicate glass substrate illuminated by an integrating sphere and photograped. Un-treated tube on the top and cleaned tube on the bottom. . . . . . . . . . . . . . . . . . 54

5.12 Cracks in PMMA after contact with acetone for about a minute. Residue of the solventand plastic was whipped of the tube before the photograph was taken. Acetone attacksPMMA faster than most solvents do, however even with toluene these cracks couldbe observed within minutes. Considering a 1 m tube and the normal coating velocity,exposure times are longer than that. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.13 The same efficiency measurement with the PMMA tube turned along the symmetry axisin between. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.14 Illuminated PMMA tube coated with P61 on the far end for about 5 cm. A differenceins light scattering behavior can be observed. . . . . . . . . . . . . . . . . . . . . . . 56

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List of Figures

5.15 The efficiency of the WLS is shown normalized to spectralon and corrected for lensabsorption. The large error is caused by low light intensity and the large amount ofmeasurements necessary to obtain this result. . . . . . . . . . . . . . . . . . . . . . . 57

5.16 Here is the spectrum taken at different positions of a coated tube shown. You cans seethe difference in shape at about 410 nm. This is the part where re-absorption takes place. 57

5.17 Comparison of the expected efficiency based on Fig. 5.15 and known losses, to themeasured efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.1 Basic efficiency of WOM and DOM. For the WOM this includes the product of paintefficiency and the convolution of PMT efficiency with the paint emission spectrum. Forthe DOM the efficiency for light coming under α = 0. . . . . . . . . . . . . . . . . . 60

6.2 Angular acceptance for the WOM and the DOM, calculated according to eq. (6.5) andmeasured [23], respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.3 WOM and DOM efficiency weighted by the Cherenkov spectrum, detector area, angularacceptance and in case of the WOM transmission improvements due to surface texturing. 62

A.1 Benzene ring and its bonding structure on the top and left (modified version of [54]).Chemical structure of Bis-MSB (1,4-Bis-(2-methylsteryl)benzene), a WLS, on the right,showing the importance of C=C bounds for the wavelength shifting process. . . . . . . 65

A.2 Predicted interaction lengths for neutrino interactions on nucleon targets: dotted line,charged-current interaction length; dashed line, neutral-current interaction length; solidline, total interaction length. The dot-dashed curve shows the charged-current interac-tion length based on the EHLQ structure functions. Lmathrmint is given in the water-equivalent interaction length per cm. [6] . . . . . . . . . . . . . . . . . . . . . . . . . 66

A.3 Transmittance of quartz glass and borosilicate glass in comparison. Both are correctedfor Fresnel losses (section 2.2.3). Data was taken by [55] on glass samples intendedas casing for the WOM. Unfortunately no errors are available. However size of entriesgrater than one might be an indication for the error size. The non physical values (> 1)might also be caused by the focusing effect of a denser medium. . . . . . . . . . . . . 67

A.4 Relative transmission spectrum of a lens with focal length f = 50 mm. . . . . . . . . . 67A.5 Efficiency measurement of PT in PMMA / anisole paint as used in Fig. 5.2 and corres-

ponding spectrum. However in comparison to the plots in Fig. 5.3 a borosilicate tubewith 20 mm diameter and 1.0(5) mm wall thickness was used. As tested in differentmeasurements the efficiency on borosilicate tubes is about a factor two to three lowerthan on PMMA or quartz glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A.6 Angular capture efficiency of the PD used in all measurements conducted in this thesis.Obtained by shining approximately parallel light on to the PD while turning it on thespot. Afterwards it was corrected for projection effects. Unfortunately the DAQ forangular measurements does not support the determination of a statistical error. . . . . . 69

A.7 Efficiency for three potential PMT’s to be used in the WOM. UBA and EG are ultrabialkali and enhanced green ultra bialkali PMT’s respectively from Hamamatsu [27].GaAsP corresponds to the R9792U-40 MHP0119, also from Hamamatsu. [52] . . . . . 70

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List of Tables

4.1 Results of adhesion tests with commercial paint on microscope slides. Different treat-ments have been applied with each two samples at two different thicknesses. The timebetween coating and first signs of paint peeling has been observed. All samples exceptthe untreated one are cleaned with ethanol and isopropyl before their individual treat-ment. For samples with peeling times > 120 no peeling has been observed yet. Forall other times the mean of the two values has been obtained with its correspondingdifference when not both samples started peeling at the same time. However the timesbetween observations have been bigger on occasions. Usually a error of five days has tobe considered. Only in the first five days it was observed daily. . . . . . . . . . . . . . 39

5.1 Results from solvability tests with each based on 50 ml of the named solvent and PT. Allsamples were heated to temperatures close to the boiling point of the solvent in order todissolve PT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.2 Result of a solvability test of different WLS in 40 ml of anisole. The weights are knownto an accuracy of at least 10−4g therefore no errors are shown. . . . . . . . . . . . . . 44

5.3 Composition of the paints presented in Fig. 5.5. From each solvent 100± 5 ml are used.The errors on the weights are smaller than the shown values. Exception is the PMMAin toluene because it was only partially solved and the remainders removed from thesolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.4 Composition of the paints presented in Fig. 5.6. For each solvent the amounts are scaledto 100 ± 5 ml. The errors on the weights are smaller than the shown values. . . . . . . 49

A.1 Velocity calibration of the dip coater. All measurements are done for a distance of59.5 ± 0.2 cm. The time has been stopped by hand and is therefore estimated to be ±1s.Compared to the smallest measured time this is about 0.4 %. . . . . . . . . . . . . . . 66

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Acknowledgements

I would like to thank Marek Kowalski for giving me the opportunity to work on this topic.Furthermore I would like to thank him and Sebastian Boser for the support and constructive ideas duringthis thesis.Also I thank Ian Brock for agreeing to be my co-corrector as well as for supplying the thesis guide [56],which saved a lot of ground work whilst writing this thesis.Moreover, I would like to thank Daniel Kuesters and Simona Lombardo for the occasional brain storm-ing and a nice atmosphere in the shared laboratory.More thanks goes to Peter Falke who actually mixed the P64 paint under my instructions while I wasalready busy writing my thesis.Spacial thanks goes to my grandmother for supporting me during my studies. (Danke Oma!)And finally I would like to thank Sebastian Boser, Jan Stillings, Andreas Homeier, Lukas Schulte,Markus Voge and Isabelle Boventer for reviewing my thesis, even at short notice. I hope at least one ofyou ever manages to read to this point.

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Declaration of Independence

I hereby declare that this thesis was formulated by myself and that no sources or tools other than thosecited were used.

Bonn, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Date Signature (Dustin Hebecker)

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