Interaction of Gamma Radiation with Matter

As in the case of charged particles (e.g., electrons, protons, a-particles), interaction of photons of y-radiation with matter is of electromagnetic nature. However, the exact physical mechanism of that interaction is quite different than that in the case of charged particles because of the following  

Photons do not have electric charge therefore, they do not participate in Coulomb interaction. Photon interaction cross section is much smaller than interaction cross sections of charged particles. 

The photon rest mass is zero therefore, their velocity is always equal to the velocity of light. That is, photons cannot be slowed down in matter (unlike charged particles). Photons can be only scattered or absorbed. 

Photon absorption is an interaction process when the photon disappears, and all its energy is transferred to atoms of the material or to secondary particles. Photon scattering is an interaction process when the photon does not disappear, but changes the direction of its propagation. In addition, the scattered photon may transfer a part of its energy to an atom or an electron of the material. There are two interaction processes whereby a photon is absorbed and several types of scattering (of which one type is much more important than the others). 

FIGURE 1.6 Compton scattering by a weakly bound electron. 

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In a gamma spectrum some other lines can also be found that are not directly related to the decay process but are due to the interaction of gamma radiation with matter. This interaction is a rather complex process. Some knowledge of the interaction will help to understand the structure of a gamma spectrum. 

Radioactive sources and particle accelerators are used to initiate polymerizations. Electrons, neutrons, and a-particles (He2+) are particulate radiations, while gamma and X rays are electromagnetic radiations. The interactions of these radiations with matter are complex [Chapiro, 1962 Wilson, 1974]. The chemical effects of the different types of radiation are qualitatively the same, although there are quantitative differences. Molecular excitation may occur with the subsequent formation of radicals in the same manner as in photolysis, but ionization of a compound C by ejection of an electron is more probable because of the higher... 

Many factors affect a gamma radiation measurement (Knoll, 1979 Tsoulfanidis, 1983). The most important factors relevant to the CAPTF are the characteristics of the radioactive source, the interaction of gamma rays with matter, the position of the source relative to that of the detector, the efficiency of the scintillation detector, and the dead-time behavior of the whole measurement system. These factors are separately discussed in the following subsections. 

Radioactive tracer techniques have long been used to study particle motion in solids fluidization systems. The advantage of this technique is that the flow field is not disturbed by the measurement facility and, therefore, the measurement of the motion of the tracers represents the actual movement of particles in the system. The tracer particles are usually made of gamma-emitting radioisotopes, and their gamma radiation is measured directly by scintillation detectors. Factors that affect gamma radiation measurement were identified as the characteristics of the radiation source, interactions of gamma rays with matter, the tracer s position relative to the detector, detector efficiency, and dead time of the measurement system. 

To summarize, the major part (>90%) of the absorbed energy of gamma radiation consecutive to primary interaction with matter is, at a definite moment, to be found as secondary electrons this fraction increases as the atomic number of the elements from the target decreases. 

Interactions of Radiation with Matter 2.4.1 Gamma Ray Interactions... 

Each of the types of radiation has a characteristic way of interacting with matter and transferring its energy. Alpha radiation has the least penetrating power and its effects are limited to the surface layers of a material, so it only needs to be considered when a surface is contaminated by an alpha emitter. Beta radiation has a range of up to a centimetre or two whilst X-ray, gamma... 

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