XV. KLKrrRONS, MUTTRONS, AND ALPHA PAHTICLES 533 



allli()uj>;h llicsc nuclei ahsorl) less energy ironi liie rKnitron at each 

 coliisicni, they do not react to any appreciable extent with theiinal neu- 

 trons. For this reason they are used as "moderators" in nuclear fis- 

 sion piles. The recommended way of providing beams of thermal 

 neutrons for biological experiments is therefore to allow a beam of fast 

 neutrons to pass into a mass of very pure graphite or heavy water. 

 Prior to the construction of "piles" only two biological experiments 

 with slow neutrons were reported (70,71). In each case the fast 

 neutrons from a cyclotron were slowed down by passage through wax 

 or water. Specimens impregnated with lithium or boron were ex- 

 posed with the object of looking for a biological effect due to ionizing 

 particles resulting from the disintegrations: 



Li« + ?!' (slow) > H''* + He* + I. .5 m.c.v. (5) 



B'o + /(.' (slow) > Li^ 4- He' + 2.r) in.c.v. (.see 72) (6) 



Thermal neutrons will in general be accompanied by rather more 

 energetic neutrons. Fortunately, however, the thermal neutrons 

 are strongly and selectively absorbed in cadmium so that an effect of 

 the thermal neutron alone can be looked for as a difference. It seems 

 that, at least in the experiments of Kruger, a biological effect of the 

 disintegration particles was established. 



Very little indeed has been published concerning the biological 

 effects of thermal neutrons on untreated tissues. Practically the 

 whole of the ionization produced in biological specimens a milli- 

 meter or so in thickness exposed to pure thermal neutrons will be pro- 

 duced by protons resulting from the disintegration of the nitrogen 

 content of the tissue: 



X'4 + n> » ('" + /> + 0.()2 (7) 



When, however, thermal neutrons fall on a layer of tissue a few centi- 

 meters in thickness the reaction with hydrogen, referred to above, 

 namely : 



II' + «' > H' + III' + 2.2 (8) 



will generate 2.2 m.e.v. y ra\^s, which will produce more ionization in 

 the tissue than the disintegration of nitrogen. For many types of 

 biological reaction, particularly the types leading to the gross in- 

 jury of somatic cells, the proton ionization, being of high linear ion 

 density, will be much more effective than an equal amount of 7-ray 

 ionization. Allowing a factor of ten for this intrinsic biological ef- 



