This document discusses scintillation detectors and their properties. Scintillation detectors work by emitting light when exposed to radiation, and this light output can be used to measure incident radiation. The key properties of scintillation detectors are that they must have high scintillation efficiency, light yield proportional to deposited energy, short decay time, transparency to emitted wavelengths, and be able to be made in large sizes and desired shapes. Common inorganic scintillators discussed are NaI(Tl), which is widely used due to its availability and high detection efficiency, and BGO, which has high intrinsic efficiency for high gamma energies.

2.  Some substances emit light when radiation fall on them.
Emission process is called fluorescence or
phosphorescence. Light output can be used as a
measure of incident radiation. This is the principle of
scintillation detectors
 Fluorescence: Prompt emission of visible radiation from
a substance following its excitation
 Delayed Fluorescence: Delayed emission of visible
radiation from a substance following its excitation
 Phosphorescence: Emission of longer wave length.
Lighter than fluorescence
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3.  High scintillation efficiency
 Light yield should be proportional to the deposited
energy
 Detector material should be transparent to the
wavelength of its own emission
 Decay time of the induced luminescence should be
short
 It should be possible to make them in large size and
desired shape
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4.  Refractive index should be near to that of glass (1.5) to
permit efficient coupling with the photo multiplier tube
 High density and high atomic number
 Good temperature stability and mechanical properties
 Good resolution
 Ease of operation
 Non hygroscopic
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5.  Inorganic Scintillator
 Organic Scintillator
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Both inorganic & organic Scintillators
can be used as detectors

6.  Alkali halides (NaI(Tl)) are the most widely used for
this purpose
 They are insulators and have wide gap between the
valence band and conduction band (NaI)
 Suitable activators (Tl) are used to create excited
states which decay by emission of light in the visible
range
 Decay time = 230 ns (91%), 0.15 sec (9%)
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NaI(Tl)

7.  Scintillation efficiency ~13%
 All photons do not reach photocathode of PM tube 
light collection efficiency <<100%
 Quantum efficiency of photocathode << 100%
 661 keV gamma photon will give ~2000 photoelectrons
 Resolution =100x2.35/2000 ~ 6%
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NaI(Tl)
Activator sites

8.  NaI(Tl) is an alkali halide inorganic scintillator
 High Z from iodine (53)
 This results in high efficiency
 Resolution is around 6%
 NaI(Tl) detector is the most widely used
scintillator for gamma counting due to its
availability in desired size and shape and high
detection efficiency
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9. Bismuth germinate (BGO) (Bi4Ge3O12) is a
scintillation crystal with high Z of bismuth (83)
 High intrinsic efficiency at high gamma ray
energies
 Excellent alternative to NaI for gross counting
applications
 Poor energy resolution
9
LaBr3 (Ce3+) gives an energy resolution of
2.9% for the 661.7 keV

10. Nuclear Detectors 10
Bismuth Trisulphide (Bi2S3) ZnS(Ag)
Gallium Selenide (GaSe) CaF2(Eu)
Lead Iodide (PbI2) CsF
Glass Scintillators BaF2
Scintillation gases CsI(Na)
LiI(Eu) (neutron detection) AlSb
CsI(Tl) has a higher intrinsic efficiency than
NaI(Tl) but less than BGO

11. Liquid Scintillators:
 These detectors find wide application where
large volume detectors are required
 They are also used for simple alpha and beta
counting
 100% efficiency for alpha
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Anthracene is one of the oldest organic material used
for scintillation purposes and has high scintillation
efficiency
Stilbene is also widely used but it has low scintillation
efficiency

12.  Solvent = Dioxan (1 litre)
 Scintillator = PPO (0.7%) (2,5 diphenyl oxazole)
 Wavelength shifter = POPOP (0.03%)
(1,4 bis-[2-(5-phenyloxazolyl)]-benzene
 To keep Pu in the complex form = TOPO (2%)
 Anti quencher = Naphthalene (10%)
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13. Nuclear Detectors 13

14.  Fast timing response, they are very useful in
nuclear spectroscopy
 Poor energy resolution due to large energy
required for one photon/electron
 Used as anti-coincidence detector to reduce the
background or to reduce pile up effects
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15. Nuclear Detectors 15

16. Nuclear Detectors 16

17. D = Compton Valley (Between Full Energy Photopeak and Compton Edge)
E = Backscatter Peak (Gamma ray scattering from the surrounding medium)
F = Excess Energy Region (Compton events from high energy gamma rays, Pile
up effect)
G = Low Energy Rise (Electronic noise)
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A = Full Energy Photopeak (It
centeroid represent the photon
energy E0, Width is due to
statistical fluctuations)
B = Compton Background
Continuum (Below Compton
Edge)
C = Compton Edge (Ec) (It
represents the maximum
energy that a photon can
transfer in a single scattering
event
Fig 3.7 Page 53
Photopeak
Compton
Edge
Backscatter