ABSTRACT
Particles and waves are distinguished only in classical mechanics. In quantum mechanics they are interchangeable, giving rise to first and second quantizations, respectively. However, it is customary to group the incident radiations as light (electrons, muons, xand )I-rays, etc.) or heavy (protons, a-particles, fission fragments, etc.) particles. Photons, classified as a separate group, can cause excitation and ionization in a molecule. Ionization by photons can proceed by any of the three following mechanisms: photoelectric effect, Compton effect, and pair production. In the photoelectric effect, the photon is absorbed and the emergent electron inherits the photon energy reduced by the sum of the binding energy of the electron and any residual energy left in the resultant positive ion (Einstein equation). In the Compton effect, the photon is scattered with significantly lower energy by a medium electron, which is then ejected with the energy difference. In the pair production process, the photon is annihilated in a nuclear interaction, giving rise to an electron-positron pair, which together carries the photon energy lessened by twice the rest energy of the electron. This process therefore has an energetic threshold. With increase in photon energy, the dominant interaction changes from photoelectric to Compton to pair production. For
60Co_')' rays, almost the entire interaction is induced by the Compton effect which, by the Klein-Nishina theory, gives a nearly flat electron energy spectrum. Thus the 1.2-MeV photon generates a wide spectrum of Compton electrons having an average energy of -0.6 MeV; consequently, the medium molecules respond as if being showered by these fast electrons. Pair production is a characteristic of very high energy photons; again, to the medium molecules, the effect is the same as a spectrum of fast electrons and positrons of appropriate energy. Electrons of various energy are the most important sources as primary radiations in the laboratory and in the industry [1], and also as secondary radiations in any form of ionizing event. Other often-used irradiations include x-rays, radioactive radiations (u, r?>, or ')'), protons, deuterons, various accelerated stripped nuclei, and fission fragments. X-rays differ from ')'-rays operationally, i.e., x-rays are generated by machines whereas ')'- rays are produced in nuclear transformations.
