534 L. H. GRAY 



fectiveness, Mitchell (73) calculates that when a large animal is ex- 

 posed to a beam of thermal neutrons disintegration of nitrogen will 

 account for about 60% and the y rays for about 40% of the total bio- 

 logical effect in the superficial cells. 



The nuclear fission pile provides larger fluxes of thermal neutrons 

 than any other source; Zirkle {67) has estimated the acute lethal 

 dose of thermal neutrons for mice by exposing animals in a pile. The 

 animals were introduced through a tunnel in the pile shield to a posi- 

 tion at which they would be exposed to a large flux of slow neutrons. 

 The exposure chamber was made of bismuth to absorb y rays, and 

 bismuth partitions were arranged so that each animal was shielded 

 from the y rays emitted from its neighbors as a result of the slow neu- 

 tron capture by hydrogen (reaction 8). Half the irradiated animals 

 survived three weeks after exposure to a slow neutron dose of 4.2 X 

 10^^ slow neutrons per cm.^ under the conditions of this experiment. 

 Other radiations were present and it is interesting to note that the 

 contributions to the total ionization in the tissues were estimated 

 to be: fast neutrons 45 "n" (units) (defined in Sect. D4); y rays 

 84 r.; protons from the disintegration of nitrogen 54-68 n; radio- 

 activity induced in the tissues by neutron bombardment 10 r. Since 

 previous work had shown that as regards acute lethal effects in mice 1 

 n of neutrons was about as effective as 9 r. of j8 or y radiation, the 

 four contributions to the total effect of exposure in the pile were as- 

 sessed as equal or equivalent to 45 n, 10 n, 54-68 n, and 1 n, respec- 

 tively, totaling 109-123 n, in fair agreement with the measured acute 

 mean lethal dose for fast neutrons, which had been found to be 91 n. 

 The conclusion was drawn that the results exclude the possibility 

 and the fear of the existence of some unknown mechanism that creates 

 a hazard far greater than that due to the known mechanisms. It will 

 be seen that the contribution of slow neutrons to the total effect was 

 only estimated to be 50% and, judging by the figures given for the 

 slow neutron flux, may barely have exceeded 30%. 



These figures serve to emphasize the extreme technical difficulty 

 that at present attends an investigation of the biological effects of 

 thermal neutrons obtained by the passage of fast neutrons through a 

 moderator. It is possible that slow neutrons could more easily be 

 obtained from the (D-C) reaction (reaction 4, Sect. C3). This reac- 

 tion has not hitherto been used for biological work. The yield is 

 much less than from the (D-D) reaction even at 1 m.e.v. and the 

 product nucleus, N^^, is radioactive with a half-life of ten minutes. 



