PRINCIPLES OF RADIOLOGICAL PHYSICS 69 



poorly, and even tentative guesses can hardly be made regarding the 

 relative probability of different processes in condensed materials, whereas 

 eventual negative ion formation is practically certain in gases. The 

 role of electric relaxation (see Sect. 3-5), that is, of molecular reorienta- 

 tion and deformation around a positive ion in a condensed material, 

 even prior to capture of the secondary electron, has been drawing atten- 

 tion in very recent years (see Chap. 3). 



The formation of negative ions constitutes in itself a semipermanent 

 change whose chemical and biological effectiveness is uncertain. This 

 is considered in Chap. 3, where it is pointed out that the formation of a 

 negative ion often leads to chemical dissociation. This happens, for 

 example, in water. 



Only highly electronegative atoms, or groups of atoms, such as F, CI, O, 

 and Oo, are capable of capturing electrons. Moreover, electron capture 

 proceeds slowly, at best; capture occurs, on the average, only after 

 thousands of collisions with electronegative atoms. 



3-2. ACTIVATION BY X AND GAMMA RAYS 



X or 7 rays traversing matter produce three main types of phenomena : 

 the photoelectric effect (Sect. 2-3), the Compton effect (Sect. 2-2a), and 

 pair production (Sect. 2-2b). These phenomena take place here and 

 there through any material exposed to X rays. The laws of atomic 

 physics predict the prohahility that any one of these phenomena will take 

 place in any given atom, but otherwise the actual occurrence of these 

 phenomena proceeds at random. 



Each individual process of photoelectric effect or of pair production 

 causes the disappearance of one X-ray photon. The energy of this 

 photon is transferred to one or two electrons which are thereby violently 

 ejected and constitute a new corpuscular ionizing radiation. A part of 

 the energy of the photon may be reemitted in the form of lower potency 

 X rays, namely, by X-ray fluorescence (see Sect. 2-3) following a photo- 

 electric effect, by positron annihilation following pair production (see 

 Sect. 2-2c) or by Bremsstrahlung. 



Each process of Compton effect causes the transfer of a part of the 

 energy of one photon to an electron, which is thereby violently ejected and 

 constitutes a new corpuscular radiation. The energy transferred to the 

 electron is almost invariably far in excess of the chemical range. The 

 energy which remains in the scattered X-ray photon is subject again to 

 further transformations, like the energy of the initial photon. 



All processes produced by X rays have therefore the main effect of 

 replacing X rays with fast electrons. The chemical action of X rays is 

 exerted almost entirely through secondary fast electrons, which act, in turn, as 

 described in Sect. 3-1. 



The relative frequency of photoelectric effect, Compton effect, and pair 



