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Transcript Scintillators

 When radiation interacts with certain types
of materials, it produces flashes of light
 Materials that respond this way are called
 These flashes can be collected and counted to
obtain a measure of the radiation intensity.
 Amount of flashes produced is proportional
to the energy deposited in the crystal
Early detectors
 1903 – Crookes invented a device called a
spinthariscope used to see scintillations from alpha
particle using zinc sulfide detector
 1908- Regener used diamonds to count the
scintillations of alpha particles
 1944- Photomultiplier tube was invented
 High efficiency
 Efficiency should be linear over a wide energy range
 Transparent
 Should be easily made
 Index of refraction should be close to glass
 No material fits all of these criteria
F vs P
 Flourensence- emission of visible radiation from a
material. Prompt and delayed
 Phosphoresence- emission of a longer wavelength
light but at a much slower time interval
 Good scintillator should convert most of the energy
to prompt flouresence
Organic Anthracine, Napthaline, Stilbene
 Fast response but low efficiency
 Beta and neutron detection
 Can be solid or liquid
Inorganic NaI, CsI, ZnS, HgI, BGO
 Slower response but higher efficiency
 Higher density for gamma detection
 Usually solid
 Pure crystals
 Anthracine highest efficiency of any organic
 Stilbene pulse shape discrimination
Hard to get in large sizes
Plastic Scintillators
 Organic scintillators are dissolved in a solvent and
can be polymerized
 Can easily be made in large volumes
 Inexpensive
 Have to worry about self absorption
 Efficient for low energy beta particles and x rays
 Can be in large volumes
 High efficiencies
 More Later on Liquid Scintilation process
Toxic Benzene, Toluene, Xylene
Non-toxic POP, POPOP, Ultima Gold
Other Organic scintillators
 Thin Film
 Can be used as transmission detectors
 Loaded Organic detectors
 Can add high Z material to increase efficiency of energy
conversion to light but lowers light transmission through
 Can add high neutron capture cross section material so can
detect Neutrons through the proton recoil reaction
 Valence band- bound electrons
 Conduction band- electrons that can travel within
the crystal
 Forbidden band- where electrons can not go
 Electrons jump from valence band to conduction
 Probability of conduction band e- returning to the
valence band is small, so we add activators to the
Band gap
 Band gap is the energy difference between the
valence band and the conduction band
 In conductors the band gap is 0
 In insulators the band gap is larger
 In semi-conductors the band gap is small
 Are impurities that are added to the crystal to
improve the probability of the e-returning to the
valence band and hence releasing light in a
wavelength we can detect
 Impurities create energy states that in the forbidden
zone of the original crystal giving the e- jumping off
 Sodium iodide crystals doped with thallium
Most common scintillator
generally employed for gamma and x-ray detection
Can be made large
Has excellent light production
Very hydroscopic
Linear response
Very fragile
 Cesium Iodide (CsI) with Tl or Na
 Less fragile than NaI
 Can be shaped
 Denser material
 Pulse shape discrimination properties can differentiate
between different type of radiation
 Good if need small efficient detector
 Zinc sulfide doped with silver (ZnS(Ag)) ,
well suited for alpha and heavy ion detection
Efficiency similar to NaI(Tl)
Polycrystaline form limits size
they use a large area but thin crystals for portable survey
First type of radiation detector
 Bismuth Germanate (BGO)
 Pure scintillator
 High density
 Not as fragile as NaI
 High efficiency
 Poor energy resolution
 LaBr3(Ce)- Lanthanum Bromide
High density
Good resolution
 Others
 BaF2
 CaF2
 CsF
Scintillator crystal
 Must be clear with no defects
 What would the effect on light propagation if the
crystal had a
Other than doped impurities
Photomultiplier Tube
 Device that changes a small number of photons
created in a scintillator (or other process) into a
number of electrons that can easily be counted.
 Glass enclosed, vacuum sealed components
 Shock and vibration sensitive
 Magnetic fields will effect PTMs
Photomultiplier Tube (PMT)
 Photocathode- has the unique
characteristic of producing electrons when
photons strikes its surface (photoelectric
 Dynodes- When each electron from the
photocathode hits the first dynode, several
electrons are produced (multiplication),
this sequence continues until the electron
pulse is now millions of times larger then
it was at the beginning of the tube
Photomultiplier Tube (PMT) cont
 Anode- At this point the millions of
electrons are collected by an anode
at the end of the tube forming an
electronic pulse.
 Signal – multiplied pulse sent to
other electronics for processing
 Signal collected at the anode has
been multiplied many times from
when it entered the photocathode
Photomultiplier Tube (PMT)
Incident Ionizing Radiation
Photomultiplier Tube
Light Photon
Optical Window
Several configurations
Venetian blind
Box and grid
Linear structure
Circular grid
 Venetian blind- old , slow response time, not used
 Box and grid- old and slow but is good for large PMT
 Circular grid and linear structure-faster response