Compton scattering process

Figure 1.8. The Compton scattering process in which a 7 ray transfers only a part of its energy to an electron in a shell and is itself scattered with reduced energy. The electron is ejected from the shell with energy, — Eb, where Etf is the partial energy transferred by the 7 ray and b is the binding energy of the electron in the shell. The remaining 7-ray energy appears as a scattered photon.

Figure 3.3 A typical spectrum obtained from a measurement of an iodine solution with Am as irradiation source. The characteristic iodine peaks are considerably smaller than the peak of Compton-scattered source photons. In the Compton scattering process, which involves the outer electrons, the incoming photon transfers a part of its energy to an atomic electron, which is then knocked out from the atom. The direction of the continuing photon is changed in the process, and the amount of energy loss is determined by the scattering angle.

Fig. 16. Feynmann-type diagram of the Stimulated Compton scattering process and nature of upper and lower states of Free Electron Laser transition".

In addition to Compton scattering, y-rays having energies above 1022 keV interact with matter by a process called pair production, in which the photon is converted into a positron and an electron. The y-ray energy in excess of the 1022 keV needed to create the pair is shared between the two new particles as kinetic energy. Each j3 -particle is then slowed down and annihilated by an electron producing two 511-keV photons. 

Inelastic photon scattering processes are also possible. In 1928, the Indian scientist C. V. Raman (who won the Nobel Prize in 1930) demonstrated a type of inelastic scattering that had already been predicted by A. Smekal in 1923. This type of scattering gave rise to a new type of spectroscopy, Raman spectroscopy, in which the light is inelastically scattered by a substance. This effect is in some ways similar to the Compton effect, which occurs as a result of the inelastic scattering of electromagnetic radiation by free electrons. 

The basic processes which are responsible for reaction on irradiation of these covalent inorganic systems involve excitation and ionization of the molecules by the secondary electrons generated in the sample by Compton scattering. The primary species produced are excited molecule ions... 

Figure 5 Schematic representation of neutron Compton scattering on an entangled pair of identical particles. The state is a superposition corresponding to particles a and (3 receiving the recoil during the scattering process.

In traversing through matter. X-rays are attenuated by coherent (Rayleigh) and incoherent (Compton)scattering and are absorbed by the photoelectric process (6, 7). X-rays of energy below 100 keV are mainly absorbed by the photoelectric process with a cross section (i.e., the probability for absorption) proportional to (6), where E is the X-ray energy and Z... 

Figure 1.10. Linear attenuation coefficient of 7 rays of different energies in water (equivalent to body tissue). The relative contributions of photoelectric, Compton scattering, and pair production processes are illustrated.

The detection efficiency of a detector is another important property in PET technology. Since it is desirable to have shorter scan times and low tracer activity for administration, the detector must detect as many of the emitted photons as possible. The 511-keV photons interact with detector material by either photoelectric absorption or Compton scattering, as discussed in Chap. 1. Thus, the photons are attenuated (absorbed and scattered) by these two processes in the detector, and the fraction of incident 7 rays that are attenuated is determined by the linear attenuation coefficient (/x) given in Chap. 1 and gives the detection efficiency. At 511 keV, /x = 0.92 cm-1 for bismuth germanate (BGO), 0.87 cur1 for lutetium oxyorthosilicate (LSO), and 0.34 cm-1 for Nal(Tl) (Melcher, 2000). Consequently, to have similar detection efficiency, Nal(Tl) detectors must be more than twice as thick as BGO and LSO detectors. 

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