NUCLEAR COLLISIONS 101 



probability compared to simple ionization, and thus would seem to of- 

 fer promise of importance (in the absence of chain reactions in inter- 

 mediate stages) principally in the case of reactions of low yield per "ion 

 pair" or low "target area." Of the processes one, especially, affords a 

 mechanism of great energy transfer to a small volume — perhaps a 

 single molecule— and appears to have quite appreciable probability in 

 many instances. 



Nuclear Collisions 



Some fraction of the energy of any high-energy radiation penetrating 

 a medium is lost in direct momentum transfers from charged particles 

 to atoms as whole units. The primary process here is customarily called 

 a nuclear collision, because the effective interaction is that between the 

 (screened) Coulomb fields of the particle and the nucleus of the atom. 

 An atom experiencing an impact of this type is often ejected from its 

 original position in the medium, and will then usually come to rest else- 

 w^here. If the struck atom acquires sufficiently great kinetic energy, 

 some of this may be lost in excitation and ionization; the remainder, 

 and practically the entire energy of more lightly ejected atoms, is ex- 

 pended in further nuclear collisions. Much of the energy that is trans- 

 ferred by this mechanism goes directly or indirectly into excitation of 

 molecular vibrations and is for the most part ultimately dissipated into 

 heat; some of the original energy loss, however, is preserved as aug- 

 mented potential energy of the medium, deriving from the altered atomic 

 arrangement. The nuclear collision is known to be the effective process 

 in the observed disordering of the structure of a solid substance by heavy 

 charged particles (the latter being either the primary radiation or second- 

 ary particles projected under neutron irradiation). Indeed, this effect 

 is apparently not brought about, at least with appreciable yield, by sim- 

 ple ionization events. 



For most of the range of an energetic charged particle this mode of 

 energy loss contributes a very minor fraction — for protons or alpha parti- 

 cles in media composed of light atoms roughly 0.05-0.10 per cent — of 

 the total. For very slow heavy particles, and therefore for initially en- 

 ergetic particles near the end of their ranges, it is the predominant 

 process, but the energy of these particles is so low (of the order of 1 

 kev for alpha particles) that the total energy transferred to nuclear mo- 

 tion is small. Thus, to cite an example, rough estimate shows that a 

 10-Mev alpha particle absorbed in air will lose 8 kev in nuclear colli- 

 sions, of which about 1 kev will be lost at the very end of the range. In 

 light media about two-thirds of the nuclear-collision energy loss will be 

 in collisions violent enough to remove the struck atom from its mole- 



