Scintillation Detector.pptx

Scintillation detector for gamma radiationa

1. Scintillation detectors

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 Nuclear Detectors 2 Scintillation Detectors

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 Nuclear Detectors 3 Important properties of a good scintillation detector

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 Nuclear Detectors 4 Important properties of a good scintillation detector

5. Inorganic Scintillator Organic Scintillator Nuclear Detectors 5 Types of scintillation detectors 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%) Nuclear Detectors 6 Inorganic Scintillator 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% Nuclear Detectors 7 NaI(Tl) 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 Nuclear Detectors 8 NaI(Tl)

9. Bismuth germinate (BGO) (Bi 4 Ge 3 O 12 ) 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 Other Inorganic Scintillator 9 LaBr 3 (Ce 3+ ) gives an energy resolution of 2.9% for the 661.7 keV

10. Nuclear Detectors 10 Other Inorganic Scintillator Bismuth Trisulphide (Bi 2 S 3 ) ZnS(Ag) Gallium Selenide (GaSe) CaF 2 (Eu) Lead Iodide (PbI 2 ) CsF Glass Scintillators BaF 2 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 Nuclear Detectors 11 Organic Scintillators 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%) Nuclear Detectors 12 Composition of Liquid Scintillators

13. Nuclear Detectors 13 Emission in Organic Scintillators

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 Nuclear Detectors 14 Plastic Scintillators

15. Nuclear Detectors 15 Key components of a typical Scintillation Counter

16. Nuclear Detectors 16 Typical Gamma Ray Spectrum on a NaI(Tl) detector

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) Nuclear Detectors 17 Different regions in a gamma ray spectrum A = Full Energy Photopeak (It centeroid represent the photon energy E , 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

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