IONIZING RADIATIONS 5 



2. It is by no means obvious that collisions like the glancing ones, 

 which are sometimes quite distant, can be treated as the direct inter- 

 action of only two systems: the moving particle and the struck atom. 

 Surrounding or intervening atoms may be polarized by the varying 

 electric fields. Their distorted charge distributions will in turn affect 

 the net force felt by the atom under consideration. Thus the energy 

 loss will be dependent on the chemical composition, and not simply on 

 the electron density. This effect seems rather small for overall energy 

 loss, with a notable exception in the case of very fast particles, where 

 the difference between gases and solids is very marked (4). 



3. When quantum effects play a role, it becomes clear that the simple 

 picture in which an electron is either removed from its bond entirely, 

 forming an ion pair, or is simply shaken up and allowed to return to its 

 original state without any energy gain, is not correct. In fact, the 

 energy lost by the incident particle must eventual^ appear in one of 

 three forms; excitation of the atoms or molecules of the material to 

 discrete excited states; ionization into the continuum; or kinetic energy 

 of secondaries too small to excite any more atoms. Clearly the last 

 form will be negligible in complex material like protoplasm or even water; 

 whereas the first form may be of great importance and may lead to 

 irreversible chemical change, not merely to thermal motion of the 

 stopping material. Radiation of light quanta may intervene, and is in 

 fact to be expected for fast-moving particles. 



Of the relative importance of ions and excited atoms Fano will have 

 more to say ; it is enough to observe here that for every ion pair formed 

 one \xi\\ expect two or three excited atoms. The precise kind and 

 number of excitations will in general depend on the stopping material, 

 but not much on the velocity of the particle, so long as it still moves 

 fairly fast compared to the atomic electrons. 



In spite of these difficulties we can give a pretty fair account of the 

 space rate at which energy is lost from a particle in tissue. The accom- 

 panying graph (Fig. 2) is a fair sample of the dependence on velocity 

 and charge; we summarize (Fig. 3) [after L. H. Gray (5)] the energy loss 

 per micron of path for typical radiation types. 



The energy set free on ionization may, of course, include considerable 

 kinetic energy given to the newly freed electrons. Most of this energy 

 appears in electrons of rather low energy, which are capable in their 

 subsequent motion of exciting and ionizing a few more atoms them- 

 selves. Thus very frequently the ions are produced not in single pairs 

 but in little clusters of some two to four or five pairs with the corre- 

 sponding excited atoms. Once such a secondary electron has received, 

 as it occasionally will, sufficient energy- to produce more than these few 



