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Gamma-Ray Spectra _ + The photomultiplier records the (UV) light emitted during electronic recombination in the scintillator. Therefore, the spectrum collected in the scintillation detector combines the characteristics of the emitter as well as absorptive processes in the scintillator.

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Radiation Absorption The radiation energy is being absorbed through several mechanisms. major mechanisms: other mechanisms: a)photoelectric effect b)Compton scattering c)pair production d)coherent scattering e)photodisintegration f)edge absorption

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Photoelectric Effect E = h The energy of the photon is used to ionize the atom (work function) and the kinetic energy of the photoelectron. K = h - Traveling with considerable kinetic energy, photoelectrons ionize a number of atoms.

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Absorption Cross-Section The probability of absorption is proportional to the total absorption cross-section ( in Photoelectric Effect ) Z – atomic number h – photon energy (keV) S – function of h and Z h [MeV] 0.010.11.010.0 [cm -1 ] 0.1 1.0 10 100 Absorption coefficient is NaI.

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Angular distribution The number of photoelectrons ejected into angle d making an angle with the incoming photons is where 00 30 60 90 120 150 012 1 2 = 0 = 0.5

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Compton Scattering h Conservation of momentum and conservation of energy lead to h ’ the energy of the scattered photon the energy of the electron Compton shift:

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Absorption Cross-Section ( Klein – Nishina equation (1929) ) ( in Compton scattering ) where Thompson scattering cross-section by electrons h [MeV] 0.010.11.010.0 [cm -1 ] 0.1 1.0 10 100 photoelectric Absorption coefficient is NaI. Compton

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Angular distribution The differential form of the Klein – Nishina equation gives the angular distribution of the scattered photons 00 30 60 90 120 150 20 MeV 100 keV ~0 keV

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Pair Production h > 1.02 MeV e-e- e+e+ Almost the entire energy of the photon is used for the relativistic energy of the pair. The recoil of the nucleus satisfies the momentum conservation.

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Absorption Cross-Section Approximate value valid up to 15 MeV ( in pair production ) where Thompson scattering cross-section by electrons h [MeV] 0.010.11.010.0 [cm -1 ] 0.1 1.0 10 100 photoelectric Absorption coefficient is NaI. Compton pair prod. 100

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Positron-Electron Annihilation e-e- e+e+ h h Usually annihilation takes place at low kinetic energy of the particles. Conservation of momentum favors the emission of two (511 keV) photons emitted in the opposite directions. e + + e - = 2h

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Coherent Scattering Coherent scattering results from the interference of photons scattered from a number of scattering centers. Cross-sections for coherent scattering are small, therefore it is an unimportant mechanism of absorption for radiation detection.

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Photodisintegration The absorption of photons associated with photodisintegration occurs above precise energy thresholds causing removal of a nucleon from the nucleus 9 Be + h 8 Be + 1 n (1.66 MeV) 2 H + h 1 H + 1 n (2.22 MeV) Photodisintegration is used for calibration. 1n1n High energy photons (>20MeV) used to produce neutron beams..

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Absorption Edges Photoelectric effect and Compton scattering are in resonance with the absorption edges. h [MeV] 0.010.11.010.0 [cm -1 ] 0.1 1.0 10 100 photoelectric Absorption coefficient is NaI. Compton pair prod. 100 The absorption edges correspond to the characteristic X-ray emission of the scintillator. K-edge Total K-edge

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Spectral Features - The photopeak The excitation and recombination (photoelectric effect and Compton scattering), taking place within the “life-time” of the photocathode emission, result in the main photopeak. Statistic fluctuations broaden the peak, according to approximately normal distribution. 0.5 1.0 1.52.0 MeV Cts 22 Na photopeak

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- the escape peak Some ionized atoms do not recombine in the time of the photopeak. The de-excitation becomes a delayed event and the absorption of the secondary X-ray radiation produces the escape peak. Spectral Features 0.5 1.0 1.52.0 MeV Cts 22 Na Iodine escape peak

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- Compton continuum The dissipation of Compton electron energy does not contribute to the photopeak. The energy of Compton electrons ranges from zero up to the maximum that the original photons could transfer. The band with a high-energy edge called the Compton continuum results from the dissipation of the Compton electron energy. Spectral Features 0.5 1.0 1.52.0 MeV Cts 22 Na Compton continuum

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- backscattered peak Often the scattering of the original radiation from the shielding produces a backscatter peak of energy lower than that of the photopeak. 0.5 1.0 1.52.0 MeV Cts 22 Na backscattered peak Spectral Features

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- sum peak There is positive probability for the absorption of two photons from the source within a short time interval resulting in the sum peak. 0.5 1.0 1.52.0 MeV Cts 22 Na sum peak Spectral Features

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- pair production peaks Pair production (above 1.02 MeV) adds peaks at h -1.02 MeV h -0.511 MeV 0.511 MeV Spectral Features (both particles escape from the scintillator) (one particle escapes from the scintillator) positron annihilation takes place outside the scintillator.

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