NEUTRONS AND NUCLEAR COLLISIONS 11 



Neutrons and Nuclear Collisions 



The irradiation of tissue with neutrons provides one more example of 

 the deviousness of the path by which energy transfer finally is made by 

 ionizing particles. Neutrons, having no electric charge, interact only 

 negligibly with the atomic shell electrons. They collide, then, only with 

 the nuclei; their mean free paths between collisions are measured in 

 centimeters rather than in angstroms, because of the great difference in 

 size between nuclear and atomic structure. A fast neutron collides 

 almost entirely with the H atoms of tissue, setting them into rapid 

 motion, with any energy between the incident Eq and 0. These secondary 

 protons — for they will be generally stripped by the shock of collision — 

 then move through the material, ionizing as they go. This makes protons 

 of low range important agents of biological irradiations. Inelastic 

 collisions of neutrons with nuclei are quite frequent as well, with very 

 fast neutrons and heavier atoms. The energy lost to the motion is con- 

 verted to a gamma ray or even two or three. When the neutron is 

 moving so slowly that it can no longer cause strongly ionizing recoils, it 

 still sets free neutral atoms to make more collisions, disturbing chemical 

 bonds. Even after it has dropped below the excitation region for 

 molecular transitions it will still persist until it reaches thermal equi- 

 librium. Finally it will be captured by a nucleus, in tissue generally by 

 N^*, to yield two recoil ions, a proton with less than 1 mev and a slow, 

 heavy recoil C^* atom. These projectiles are especially important for 

 thermal neutron effects, where only capture gammas and capture recoil 

 ions can transfer appreciable energy. 



High-energy protons or mesons also yield nuclear reactions. Some of 

 these will be sources of multiple highly ionizing tracks. Such star events, 

 while not major contributors to overall energy transfer, may possibly 

 turn out to be sources of specially observable effects, because a single 

 cell may with one event be heavily damaged. Even the products 

 of neutron or charged-particle nuclear reactions, which in general will 

 be radioactive nuclei, may have a role. There is some evidence that the 

 effect of the recoil energy and chemical change subsequent to a radio- 

 active disintegration of a bound atom of P^^ may have a biological 

 consequence considerably more important than that of the same ioni- 

 zation energy delivered external to the binding molecule. The possi- 

 bilities here are notable, and suggest caution in the use of energy- 

 absorption data alone as a predictor of biological effect for specific tracer 

 activities. 



Enough has been said in this very cursory survey to demonstrate that 

 even within the first hundredth of a microsecond or less, the physicist's 



