lec 3 scintillators
TRANSCRIPT
-
7/28/2019 Lec 3 Scintillators
1/33
Pakistan Institute of Engineering & Applied Sciences (PIEAS)
Course: Radiation Interaction & Detection
Dr. Nasir M Mirza
Deputy Chief Scientist,
Room # A114,
Department of Physics & Applied mathematics,
PIEAS, P.O. Nilore, 45650, Islamabad.
Email: [email protected]
Ph: 092 51 9290273 (ext: 3059)
mailto:[email protected]:[email protected] -
7/28/2019 Lec 3 Scintillators
2/33
Scintillation Detector
Lecture One:
-
7/28/2019 Lec 3 Scintillators
3/33
Text Books for the Course:
1. Glenn F Knoll s Radiation Detection & Measurement (recentedition) (this shall be the main text for the course).
2. W.J. Prices Nuclear Radiation detection, McGraw Hill BookCompany.
By: Dr. Nasir M Mirza
Office: Block A (Room Number, A -144)
Office Phone: extension, 3059Email: [email protected]
Scintillation Detector
mailto:[email protected]:[email protected] -
7/28/2019 Lec 3 Scintillators
4/33
Scintillation
Scintillation
Flash of light (Visible)
From radiation detection point of view---flash of light
produced from excited atoms as a result of radiationinteraction
Luminescence
Usually occurs at low temperatures and is thus a form
of cold body radiation.
-
7/28/2019 Lec 3 Scintillators
5/33
Scintillation
Scintillation
Detection of ionization radiation by scintillation light
produced in a material is one of the oldest technique
on record (for example ZnS screens were used
initially as detectors);
Common scintillation materials include NaI(Tl),
CsI(Tl), CsI(Na), Li(Eu), ZnS, Ca2F, BaF2, glass-
Scintillators, and plastic Scintillators;
-
7/28/2019 Lec 3 Scintillators
6/33
Scintillation (Contd.)
One of the earliest means of measuring radiation
Rutherford experiments (alpha particle scattering)
used zinc sulphide crystals as the primary detector of
radiation
Used his eye to see the flickers when alpha struck
zinc sulphide
Now-a-days Photo-Multiplier-Tube (PMT) is used;
-
7/28/2019 Lec 3 Scintillators
7/33
Photo-Emissive Processes
Fluorescence: A luminescence that is mostly foundas an optical phenomenon in cold bodies, in whichthe molecular absorption of a photon triggers theemission of another photon with a longer wavelength.
Phosphorescence: A delayed luminescence, that is, aluminescence that persists after removal of theexciting source. It is sometimes called afterglow.Corresponds to light of longer wavelength than
fluorescence Delayed Florescence: Delayed emission than
fluorescence & Light of equal wavelength tofluorescence
-
7/28/2019 Lec 3 Scintillators
8/33
Properties of Good Scintillator
It should convert the Kinetic Energy (K.E.) ofcharged particles into detectable light with a high
scintillation efficiency;
This conversion should be linear the light yieldshould be proportional to deposited energy over as
wide range as possible;
The medium should be transparent to the wavelength
of its own emission for good light collection;
-
7/28/2019 Lec 3 Scintillators
9/33
Properties of Good Scintillator (Contd.)
The decay time of the induced luminescence shouldbe short so that fast signal pulses can be generated;
The material should be of good optical quality and
subject to manufacture in sizes large enough to be ofinterest as a practical detector;
Its index of refraction should be near that of glass (n
~ 1.5) to permit efficient coupling of the scintillation
light to a PMT or other light sensor
-
7/28/2019 Lec 3 Scintillators
10/33
Properties of Good Scintillator (Contd.)
No material simultaneously meets all these criteria----the choice is a compromise
Most widely applied scintillator are
Inorganic Alkali Halides
Organic based Liquids
and Plastics
-
7/28/2019 Lec 3 Scintillators
11/33
Properties of Good Scintillator (Contd.)
Property InorganicScintillator
Organic Scintillator
Light Output yield more Less
Light emission
response
Slow fast
ZNumber high Low
Radiation spectroscopy Gamma rays Beta and neutrons
-
7/28/2019 Lec 3 Scintillators
12/33
Mode of Operation of Scintillator Detectors
Mostly used in pulse mode
Advantages of using in pulse mode are:
Setting small time constant can eliminate the
unwanted phosphorescence and delayed
fluorescence
Current mode scintillator detectors used where
radiation intensity changes rapidly and will sufferfrom after glow effects if long lived decay
components are significant
-
7/28/2019 Lec 3 Scintillators
13/33
Organic Scintillators
Fluorescence in organics is due to transitions in the energy
levels of a single molecule
And it is independent of physical state of the organic
scintillation material
Example is Anthracene where solid or vapor will do the
fluorescence.
However, inorganic Scintillators (such as NaI(Tl) do need a
regular crystal structure as a basis to emit scintillations.
-
7/28/2019 Lec 3 Scintillators
14/33
Organic Scintillators
Large category of organic
Scintillators are based on organicmolecules with some symmetryproperties
and this gives rise to -electronstructure of organic molecules;
Singlet states (spin 0): S0, S1, S2, ...
Triplet states (Spin 1): T1, T2, T3, ...
Energy spacing is generally of theorder of
S1 - S0 ~ 3 - 4 eV
S3 - S2< S2 - S1< S1 - S0
-
7/28/2019 Lec 3 Scintillators
15/33
Organic Scintillators (Contd.)
Vibrational states of molecules
S00, S01, S02 ,
S10, S11, S12
First subscript is for main energy
state and second is for vibrational
state.
S01 - S00 ~ 0.15 eV
However, the thermal energy is
0.025 eV which is much smaller
than 0.15eV so all molecules at
room temperature are at S00 ground
state.
-
7/28/2019 Lec 3 Scintillators
16/33
Types of Organic Scintillators
Pure Organic Crystals
Liquid Organic Solution
Plastic Scintillator
Thin Film Scintillator
Loaded Organic Scintillator
-
7/28/2019 Lec 3 Scintillators
17/33
Types of Organic Scintillators (Contd.)
Pure Organic Crystals
Anthracene
One of the oldest organic material having highest scintillationefficiency
Sti lbene
It has lower scintillation efficiency
Used for pulse shape discrimination
Both Materials are fragile and difficult to obtain in larger sizes
Scintillation efficiency is dependent on orientation of ionizingparticles with respect to crystal axis (20-30% directionalvariation) which spoil energy resolution
They get damaged by exposure to radiation
-
7/28/2019 Lec 3 Scintillators
18/33
Types of Organic Scintillators (Contd.)
Liquid Organic Scintillators Binary (Solvent + Scintillator)
Tertiary (Solvent + Scintillator + Wave-Shifter)
Commercially available in sealed glass containers
Detectors of any size, shape and cost can be made
Sometimes radioactive material is dissolved in L.S.
and counted with 100% efficiency (C14, H3 etc.)
Problems with L.S.-------if there is dissolved O2 then
there is quenching problem (so containers in which
L.S. is to be kept should be purged from O2)
-
7/28/2019 Lec 3 Scintillators
19/33
Types of Organic Scintillators (Contd.)
Plastic Scintillators Organic scintillator dissolved in solvent is
polymerized
Large volume solid Scintillators of low cost can be
made, but large size may cause some problem ofattenuation of light
Available as rods, sheets, cylinders etc.
Small size solid Scintillators---available as single
fiber, group of fibers----as bundles, ribbons etc.
Radiation damage may cause decrease in lightoutput or decrease in the light transmission
-
7/28/2019 Lec 3 Scintillators
20/33
Types of Organic Scintillators (Contd.)
Thin Film Scintillator Films ~ 20 g/cm2 or 10 m can be prepared
Can be used for particles of lowest range (heavy ions)
They act as transmission detectors
Applied in fast timing measurements
-
7/28/2019 Lec 3 Scintillators
21/33
Types of Organic Scintillators (Contd.)
Loaded Organic Scintillators Organic Scintillators are generally used for the
detection of alpha, beta and fast neutrons
For gamma rays detections, high Z materials areadded
By adding high Z material, photo peak efficiency canbe made relatively high, fast response can beattained and also they are of low cost as compared toconventional gamma ray scitillators
But, light output is reduced and resolution is degraded
For detection of neutrons, high neutron cross sectionmaterials (Boron, Lithium, Gadolinium) are added
-
7/28/2019 Lec 3 Scintillators
22/33
NaI(Tl) Scintillator
Pure Crystal NaI
Interaction of radiation results in
UV photons
It can operate at liquid nitrogen
temperature
Activated Crystal NaI(Tl)
Small amount (~ 10-3 mole
fraction) of Thallium is added as
an activator in high purity NaI;
Interaction of radiation------visible photons are emitted;
Operate at room temperature;
-
7/28/2019 Lec 3 Scintillators
23/33
NaI(Tl) Scintillator (Contd.)
Properties
Large ingots can be grown from high purity sodium iodide
Can be machined into different shapes and sizes
Scintillators of usual size or shape can also be fabricated bypressing small crystallites together
Excellent light yield
Small non-proportionality of scintillation response with depositedelectron energy. The departure from proportionality is mostpronounced at low energies
Crystal is fragile----can easily be damaged by mechanical orthermal shocks
Hygroscopic---deteriorate due to water absorption (thereforecanned in air tight containers for normal use);
-
7/28/2019 Lec 3 Scintillators
24/33
NaI(Tl) Scintillator (Contd.)
Decay time of scintillation pulse is 230 ns which is long for
fast timing or high counting rate applications
Phosphorescence with 0.15 s decay time which contribute9% of overall light yield
High Temperature Operation
Scintillation yield is dropped with increasing temperature,results in poor energy resolution
Decay time decreases with increase in temperature, givefaster response at higher temperatures
-
7/28/2019 Lec 3 Scintillators
25/33
NaI(Tl) Scintillator (Contd.)
Radiation Damage Effects
Reduction of transparency: caused by the creation ofcolor centers that absorb scintillation light
Interference with processes that give rise to the
emission of the scintillation light itself Radiation exposures can also induce long-lived light
emission in the form of phosphorescence that can betroublesome in some measurements
Radiation damage are often observed to be ratedependent and vary greatly with type of radiationinvolved
-
7/28/2019 Lec 3 Scintillators
26/33
Introduction of PMT
This device is used to convert
weak light signal from scintillator
(few hundred photons) to a
corresponding electrical signal
without adding a large amount of
noise
Simplified structure of a typical
PMT
Outer envelope is made of
glass; It sustain vacuumconditions inside the tube
and serves as pressure
boundary
CrystalPMT Base
-
7/28/2019 Lec 3 Scintillators
27/33
Introduction of PMT (Contd.)
Photocathode
Convert light photon to lowenergy electrons calledphotoelectrons
A few hundred in number
Charge too small to serveas electrical signal
Electron Multiplier Section (Dynode)
Provide an efficient geometryfor photoelectrons
Serve as an amplifier
Multiply the electrons 1071010
electrons per photon Output Stage Anode
Collect electrons
Give output voltage pulse
-
7/28/2019 Lec 3 Scintillators
28/33
Introduction of PMT (Contd.)
Linear Behavior
Output pulse remains
proportional to number of
original photoelectrons
Timing Information
Most of the information of
original light pulse is retained
Electrons are produced within
20 50 ns after light photon
-
7/28/2019 Lec 3 Scintillators
29/33
Photocathode
Photoemission
The conversion of incident light
photon into electrons is called
photo-emission
3 stages of photo-emission process
1.Absorption of incident photon
and transfer of energy to an
electron within the photo
emissive material
2.Migration of the electron tosurface of photocathode
3.Escape of the electron from
the surface of the cathode
-
7/28/2019 Lec 3 Scintillators
30/33
Photocathode (Contd.)
Energy requirements:
Step 1
The energy of a scintillationphoton (~ 3 eV) is absorbed
Step 2
Some of energy is lost in e ecollisions in migration process.
Step 3
The remaining should be greaterthan work function (~ 1.5 2 eV)of the material;
Min. energy of photon must begreater than the potential barrier
Surface barrier must be low tomaximize escape electrons;
Rate of energy loss of migratingelectron must be small tomaximize escape depth;
-
7/28/2019 Lec 3 Scintillators
31/33
Electron Multiplication (Contd.)
Multiplication factor
The overall multiplication factor of asingle dynode is defined as (typical value is 5):
N
gainoverall
incidentelectronsprimaryof#
emittedelectronssecondaryof#
Then the Overall gain of a PM tube having N stages is
Where, is fraction of all photoelectrons collected by the multiplier structure
(typical value is 1) and N is number of stages.
-
7/28/2019 Lec 3 Scintillators
32/33
Various Configurations
-
7/28/2019 Lec 3 Scintillators
33/33
Scintillation Detector & PMT