PRINCIPLES OF RADIOLOGICAL PHYSICS 63 



elementary processes result primarily in the production of new secondary 

 radiations. 



Knock-on collisions against external electrons lead to a larger energy 

 transfer. Most of the energy is carried away by the ejected electron. 

 The collision leaves an atom in an ionized state, that is, in a particular 

 state of chemical activation. 



Similarly, any collision which affects internal electrons results in an 

 energy transfer large as compared to the energy required for chemical 

 activations. As discussed in Sect. 2-lc, most of the energy imparted to 

 internal electrons of an atom leaves the atom again in short order, without 

 suffering any too great degradation. The electron originally ejected 

 carries away much of the energy received ; the rest comes out in the form 

 of X rays or as kinetic energy of Auger electrons. In any event, the 

 atom originally affected remains singly or multiply ionized, that is, it 

 undergoes a strong activation, but most of the energy is carried away by 

 new secondary ionizing radiations. 



Radiative collisions of an electron and an atom (see Sect. l-4b) may 

 lead to the emission of a large fraction of the energy of the electron in the 

 form of an X-ray photon. This mechanism, too, has the effect of con- 

 verting energy from one kind of ionizing radiation into another one. It 

 contributes to the degradation of the radiation energy but does not dis- 

 sipate much of the energy in chemical activation. 



Sample data on the frequency of different kinds of collisions and on 

 their energetic import have been given in Eqs. (20) ff. and in Figs. 1-37 

 and 1-38. Those data pertain to the collisions within layers of matter 

 consisting of a single kind of atoms or molecules. When a material con- 

 sists of different chemical substances, the numbers of collisions affecting 

 each substance along the path of a particle may be calculated as though 

 the other substances were not there. Figures 1-39 and 1-40 give some 

 data on the course of energy degradation in water. 



The average fraction of energy spent by electrons in the production of 

 X-rays has been indicated in Sect. l-4b. Notice that an occasional elec- 

 tron may release nearly all its energy in the form of a single photon even 

 when the average X-ray production by a large number of electrons is 

 small. The X-ray production accounts for most of the energy dissipa- 

 tion by high-energy electrons traveling through materials of high atomic 

 number. The degradation of energy proceeds then mostly through the 

 process of "cascade showers," which is described in Sect. 3-2. 



3-la. Characteristics of Glancing Collisions. As we have seen, fast 

 charged particles transfer energy to atoms in packets of chemical activa- 

 tion size mostly through collisions against the external electrons, which 

 may be loosely referred to as "glancing" collisions. The other types of 

 collision transfer energy primarily from one radiation to another; the 

 energy thus transferred is eventually dealt out to atoms in packets 



