XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 493 



Neutrons that have an energy equal to the mean energy of agita- 

 tion of a molecule at room temperature are called thermal neutrons. 

 Nuclei recoiling from impact with such neutrons have of course in- 

 sufficient energy to ionize or even to excite other atoms. Each ther- 

 mal neutron, however, ultimately becomes incorporated into the 

 structure of a nucleus, which it renders unstable. The subsequent 

 nuclear readjustment is accompanied by the emission of 7 radiation, 

 by nuclear disintegration, or both. Such interactions are highly 

 specific and therefore their biological effect may depend critically on 

 the composition of the tissue irradiated. Moreover, special means 

 have to be adopted for the production of beams of thermal neutrons 

 in sufficient intensity for biological research. The unscreened nu- 

 clear fission "pile" is one such source. Thus, although the passage 

 of thermal neutrons through living matter does not give rise to ioniz- 

 ing particles essentially different in any respect from electrons, a par- 

 ticles and the particles generated by fast neutrons, it will be conven- 

 ient to consider them separately. 



Neutrons that have an energy of a few electron volts are more 

 energetic than thermal neutrons and are often referred to as epither- 

 mal neutrons. From the biophysical standpoint their distinguishing 

 feature is that they generate recoil atoms, which would be expected 

 to give rise to an abnormally high proportion of excitation relative 

 to ionization in the molecules through which they pass. For this 

 reason, it would be of great interest to investigate the biological ef- 

 fects of such neutrons. No such investigations have so far been re- 

 ported, doubtless on account of the technical difficulties that would 

 be encountered in planning such an investigation. 



In this chapter the term fast neutron will be reserved for neutrons 

 of energy greater than about 500 e.kv. (electron kilovolts). It will 

 be understood that what is said of these neutrons will require pro- 

 gressively increasing modification as the energy of the neutrons under 

 consideration falls from this value to that of thermal neutrons through 

 the intermediate regions of slow or epithermal neutrons. 



2. Effects of Ionizing Radiations on Living Cells 



For the most part electrons, neutrons, and a particles have been 

 used in order to obtain a better understanding of the mode of action 

 of ionizing radiations on living cells rather than applied as tools for the 

 investigation of purely biological problems. If delivered in sufficient 

 dose these radiations interfere with almost every known function of 



