PRINCIPLES OF RADIOLOGICAL PHYSICS 67 



whole, in ordinary phenomena of radiobiology because most of the radiation 

 energy is dissipated by fast particles. Dissipation by slow particles is of impor- 

 tance under special circumstances, especially with regard to the action of neutrons. 



"Slow" heavy particles dissipate most of their energy by forcing atomic nuclei 

 to recoil as a result of "elastic" collisions. Even though the particle is slower 

 than the atomic electrons, it may possess a momentum and energy more than 

 adequate to force its way through the atoms of a material. The atomic elec- 

 trons do not have sufficient mass to deflect the incident particle, but the atomic 

 nuclei do. Therefore the elastic collision reduces, in essence, to a Rutherford 

 scattering of the incident particle by the nucleus of the atom (see Sects. 2-4 and 

 2-2c). As the kinetic energy of the incident particle becomes comparable to 

 the binding energy of the internal atomic electrons within the material, the 

 penetration of the particle through the atoms becomes more difficult. Thus an 

 increasing portion of the atomic volume behaves as an impenetrable body, and 

 the elastic collisions acquire progressively the character of bodily impacts such 

 as take place among the atoms and molecules of a gas. 



The rate of energy dissipation by elastic collisions along the path of a particle 

 is rather low. It can be estimated by the same methods as the rate of dissipation 

 by inelastic collisions. Figure 1-40 gives some illustrative data. 



The energy dissipated through elastic collisions tends to escape detection by 

 ordinary radiation measuring devices such as ionization chambers, cloud cham- 

 bers, and Geiger counters. Nevertheless, this energy is imparted directly to the 

 motion of the massive portions of atoms (the nuclei) with respect to the adjacent 

 atoms. Therefore it may more easily cause a substantial dislocation in the 

 structure of matter and thus may turn out to be particularly effective. 



It should also be noticed that "slow" heavy particles tend to pick up elec- 

 trons, as they traverse atoms, up to the point of becoming electrically neutral 

 atoms. For example, an a particle which picks up two electrons becomes a 

 neutral helium atom. The capture of electrons by positively charged radiation 

 particles reduces their charge and thei'eby reduces their chance of producing 

 inelastic collisions. Nevertheless, it does not reduce, in essence, their dissipation 

 of energy by elastic collisions once the particles are quite slow. 



Under what conditions can an electron travel along attached to a positive ion 

 without being stripped off in the course of collisions with atoms? Consider, for 

 e.xample, an electron which is combined with an a particle to form an He+ ion. 

 This electron "sees" the atoms of matter rushing on toward it with a speed equal 

 to the speed of the He"*" ion. Therefore, if the He"*" ion as a whole moves faster 

 than the electron within the ion, the collisions with matter are "fast," as far as 

 the electron is concerned, and lead soon to a stripping of the electron. An He"*" 

 ion which moves along as fast as its electron moves about the nucleus has a 

 kinetic energy of 0.4 Mev. An a particle of this energy is liable to pick up an 

 electron and carry it along through several collisions. An a particle of much 

 lower energy normally holds on to an electron and travels as an He"*" ion. At still 

 lower energy the particle picks up a second electron. 



Observations show that the density of ions along the tracks of comparatively 

 slow, heavy, charged particles varies at intervals, indicating a change of the net 

 charge carried by the particle. For example, the charge of an oc particle of less 

 than 1 Mev appears to jump back and forth between 2e and e and eventually 



