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Gamma-Ray Spectra

_. +. 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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Gamma-Ray Spectra

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  1. _ + 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.

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

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

  4.  [cm-1] 100 10 1.0 0.1 0.01 0.1 1.0 10.0 h [MeV] Absorption Cross-Section ( in Photoelectric Effect ) The probability of absorption is proportional to the total absorption cross-section Z – atomic number h – photon energy (keV) S – function of h and Z Absorption coefficient is NaI.

  5. 90 60 120 30 150 0 0 1 2 2 1 where Angular distribution The number of photoelectrons ejected into angle d making an angle  with the incoming photons is  = 0.5  = 0

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

  7.  [cm-1] 100 10 photoelectric 1.0 Compton 0.1 0.01 0.1 1.0 10.0 h [MeV] Absorption coefficient is NaI. Absorption Cross-Section ( in Compton scattering ) ( Klein – Nishina equation (1929) ) where Thompson scattering cross-section by electrons

  8. 90 60 120 ~0 keV 30 150 20 MeV 0 100 keV 20 MeV Angular distribution The differential form of the Klein – Nishina equation gives the angular distribution of the scattered photons

  9. e- e+ Pair Production h > 1.02 MeV 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.

  10.  [cm-1] 100 10 pair prod. 100 photoelectric 1.0 Compton 0.1 0.01 0.1 1.0 10.0 h [MeV] Absorption coefficient is NaI. Absorption Cross-Section ( in pair production ) Approximate value valid up to 15 MeV where Thompson scattering cross-section by electrons

  11. e- e+ h h Positron-Electron Annihilation e+ + e- = 2h 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.

  12. 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.

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

  14. K-edge K-edge  [cm-1] Total 100 10 pair prod. 100 photoelectric 1.0 Compton 0.1 0.01 0.1 1.0 10.0 h [MeV] Absorption coefficient is NaI. Absorption Edges Photoelectric effect and Compton scattering are in resonance with the absorption edges. The absorption edges correspond to the characteristic X-ray emission of the scintillator.

  15. Cts 1.5 2.0 MeV 1.0 0.5 - The photopeak Spectral Features photopeak 22 Na 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.

  16. Cts 1.5 2.0 MeV 1.0 0.5 - the escape peak Spectral Features Iodine escape peak 22 Na 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.

  17. Cts 1.5 2.0 MeV 1.0 0.5 - Compton continuum Spectral Features 22 Na 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.

  18. Cts 1.5 2.0 MeV 1.0 0.5 - backscattered peak Spectral Features 22 Na backscattered peak Often the scattering of the original radiation from the shielding produces a backscatter peak of energy lower than that of the photopeak.

  19. Cts 1.5 2.0 MeV 1.0 0.5 - sum peak Spectral Features 22 Na 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.

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

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