PRINCIPLES OF RADIOLOGICAL PHYSICS 61 



Many radiations — X rays, for example — have a potency greatly in 

 excess of that required to produce chemical changes. These radiations 

 are easily capable of ejecting electrons from all kinds of atoms, i.e., of 

 forming ions, and are therefore called ''ionizing radiations." An X-ray 

 photon may transfer a considerable share of its energy to an electron in a 

 process of Compton scattering. After this process both the photon and 

 the electron constitute an ionizing radiation. All ionizing radiations 

 undergo repeated processes here and there in various atoms as they 

 traverse matter. As a result, their energy is progressively broken down 

 into smaller and smaller packets, i.e., it is "degraded." 



Energy packets of the order of magnitude of 1 to 10 ev suffice to 

 activate chemical changes. Therefore the energy of ionizing radiations 

 is most effective, for the purpose of activating the largest number of 

 atoms, when subdivided into packets of this order of magnitude or, as 

 we shall say loosely, when it has been "degraded to the chemical range." 



The degradation of the energy of ionizing radiations down to the 

 chemical range constitutes a complicated but rather well-understood 

 process. Most of the following discussion deals with this process. All 

 ionizing radiations turn out to produce the large majority of activations 

 through the same mechanism. Therefore all ionizing radiations produce, 

 on the whole, strikingly similar effects. 



The potency of light falls in the lower portion of the chemical range. 

 Infrared light has a low chemical potency because its photons amount to 

 approximately 1 ev only. The potency of visible light is a little higher, 

 and that of ultraviolet light is considerably higher. Light in the far- 

 ultraviolet range of the spectrum (beyond 1500 or 2000 A) has full 

 chemical potency but low practical importance because it is absorbed too 

 strongly and therefore must be handled in a vacuum (see Sect. l-3c). 



Since light has no excess potency, chemical action must be exerted 

 directly, or nearly directly, without many intervening processes which 

 would cause loss of potency. 



Radiations whose potency lies initially, or has subsequently fallen, 

 below the chemical range can still produce chemical changes but with 

 low efficiency. In fact thermal energy from any source may occasionally 

 happen to concentrate upon one or a few atomic particles in amounts 

 adequate to produce a chemical change. This concentration constitutes 

 a statistically unlikely accidental reversal of the general process of 

 degradation of energy. The chance of such unlikely events depends 

 greatly upon the absolute temperature of the material. Small accidental 

 concentrations are far less unlikely than larger ones. Because of the 

 unlikelihood of accidental concentrations, thermochemical transforma- 

 tions proceed slowdy as compared to the speed of reaction of atoms acti- 

 vated by radiation, even though they may appear fast on a macroscopic 

 scale of time. 



