Scintillators
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Transcript Scintillators
Scintillators
Scintillators
When radiation interacts with certain types
of materials, it produces flashes of light
(scintillation)
Materials that respond this way are called
scintillators.
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
Characteristics
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
Scintillators
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
Organic
Pure crystals
Anthracine highest efficiency of any organic
Stilbene pulse shape discrimination
Fragile
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
Liquid
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
material
Can add high neutron capture cross section material so can
detect Neutrons through the proton recoil reaction
Inorganic
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
band
Probability of conduction band e- returning to the
valence band is small, so we add activators to the
crystal
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
Activators
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
points
Inorganics
Sodium iodide crystals doped with thallium
(NaI(Tl))
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
Inorganics
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
Inorganics
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
instruments
First type of radiation detector
Scintillators
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
Crack
Cloudiness
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
effect)
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
Pulse
Measuring
Device
-
Sodium-Iodide
Crystal
Dynode
Photocathode
Optical Window
Anode
PMT
Several configurations
Venetian blind
Box and grid
Linear structure
Circular grid
Types
Venetian blind- old , slow response time, not used
much
Box and grid- old and slow but is good for large PMT
Circular grid and linear structure-faster response
time