Lecture 3-Building a Detector (cont’d)

George K. Parks

Space Sciences Laboratory

UC Berkeley, Berkeley, CA

Summary of Lecture 1 and 2

Photocathode

Photoelectron Emission Process

Photocathodes vs Spectral Emission of scintillators

Transmittance of window material of PMT

Dark Current

<1 photo-e- 2 photo-e-

1 photo-e-

Dark current (cont’d)

Temperature Characteristics of Dark Current

Linearity

• Non-Linearity starts when anode current exceeds 10-5A.

Spatial Uniformity

Magnetic field effects

• Magnetic field deflects electrons in PMT.

• To reduce magnetic effect, shield PMT with -metal.

• Why peak not at 0?

Unit (magnetic field mT)

Incident photons and PMT output

Pulse Height Distribution (Energy Spectra)

~5.9 keV

~662 keV

Energy Resolution

€

ΔEE

=C−AB

×100%

E

dN/dE

Energy Resolution for Scintillation Detector

€

ΔEE

=ΔNN

= NN

= 1N

Quantum Efficiency (Bi-Alkali)

Summary (Scintillation + PMT)

High energy charged particles

Comparison NaI(Tl) vs Ge(Li)

Pulse height Spectrum of Ag

Band Model

Solid state detectors

Solid State Detectors (cont’d)

Operation of semiconductor detector

Calibration of Detectors

• The amount of energy required to produce electron-ion pair in Si detectors is 3.5 eV. Hence, if we know how many electron-ion pairs are produced, we obtain the energy of the particle.

• Detectors are biased so electrons and ions are collected separately at anode and cathode.

• A detector is calibrated with known beam energies. The size of the pulse measured is directly related to the original particle energy.

• The distribution of pulse-height vs energy gives the differential energy spectrum

Detector resolution vs energy

Energy Resolution of Semiconductor DetectorsMaterial Z Bandgap Ion E (e-h) Energy Resolution

Si (77oK) 14 2.33 1.12 eV 3.61 eV 400 eV @ 60 keV

(77K) 1.16 3.76 550 eV @ 122 keV

Ge (77K) 32 5.33 0.72 2.98 400 eV @ 122 keV (0.35%)

900 eV @ 662 keV

1300 eV @ 1332 keV

CdTe(300K) ~50 6.06 1.52 4.43 1.7 keV @ 60 keV

3.5 keV @ 122 keV (2.8%)

HgI2(300K) 6.4 2.13 4.3 3.2 keV@122 keV (2.6%)

5.96 keV@662 keV

€

Mv2

2q=1

2ER

Early design of quadrispheric analyzer

• ESA designs include cylindrical, spherical and quadrispherical shaped plates.

• ESAs are basically capacitors with voltage applied across the plates.

• + and – charges are deflected in opposite directions.

• Advantages of curved plates include reducing HV (analyzer constant) and UV rejection

• Once E/q selected, particle is recorded by an electron detector CEM).

Low Energy Proton and Electron Differential Energy Analyzer (LEPEDEA)

Degradation and Noise

MicroChannel Plates (MCP)

Operation of MCP (schematic)

Degradation of MCPs

BURLE, Long-life MCP test

Background counts in detectors

A scintillation detector

• Incident photon h stops in scintillator, generates scintillation photons.• Scintillation photons propagate to photocathode and produce photoelectrons• Photoelectrons multiplied at subsequent dynodes and collected at the anode• Measured current from Anode to ground directly proportional to photoelectron flux generated at photocathode

Schematic diagram of a working Balloon-borne X-ray Detector

Summary

• Discussed briefly how to build detectors to measure Photons and Charged Particles

• Basic principle is relatively simple. There are only a few components for particle and photon detection. However, there are many details that can affect the measurements.

• Innovative ways to use these components to design one of a kind instrument to enhance science goals.

• SSTs can be used to detect Energetic Neutral Atoms (ENA).

• ESA + TOF + MCP can be used to determine M/q.

• Pin hole, coded aperture, modulation collimators yield 2D information.

• Stereoscopic view (2 SC) can yield 3D information.

Optimization

Particle measurements (Reminder)

Issues to consider in instrument design- Energy resolutions: More science with high energy resolution.

- Detector efficiency: Maintain as high as possible.

- Pulse height defect: Difference between true and apparent energy

- Channeling: effect of crystal orientation

- Dead layer: low energy threshold

- Radiation damage: degrades energy resolution and counting efficiency

- UV rejection (MCPs): spurious counts contamination

- Pulse pile up: Loss of true counts.

- Leakage current:

- Detector Noise:

- Changes with bias voltage: Effect if bias voltage is low

- Temperature sensitivity: loss of detector performance

- Cleanliness: produce noise in system.

- Micro-acoustic sensitivity:

The End

Temperature Characteristics of Photocathodes

• Temperature change is large near the long wavelength cutoff.

• Unit is in % /oC

Life Characteristics

• Damage from the last dynode due to heavy electron bombardment

Summary of PMT

• Quantum efficiency

• Collection efficiency

• Window material

• Photocathode material

• Gain

• Dark Current

• Spatial Uniformity

• Temperature Charactristics

• Magnetic field effect

• Life characteristics

• Electronic noise

• Dynamic range

• Linearity

• Time response

• Decide photon counting or measure current

• How to select photocathode material (eg, S-13, etc..)

•Less than single photon spectrum; single and two photon spectra. Important for single photon counting.

• Determine HV for operation (900, 1100, 1200, etc..)

• Determine quantum efficiency of photocathode

Channel Electron Multipliers

• ESAs usually combine

with CEMs or MCPs.

• Straight tubes produce large ion feedback current.

• Ion feedback can be reduced by curving the channeltrons Evans (1965)