INJURY AND Rl:COVER^ IN MX'TROX-IRRADIATED ANIMALS 



These data suggest that the recovery rate after irradiation by fission 

 neutrons is slower than that after y irradiation. When the data are plotted 

 on semi-log paper as in lui^urc 9, a linear fit is seen. This suggests that the 

 relationship between the unrecovered portion and the time between doses 

 is exponential, at least when the time interval is of the order of 1 to 10 

 days. 



It is seen from this figure that 50 per cent of the y-ray dose is recovered 

 Avhen the time interval between doses is approximately six days. The 



10 15 20 



Days after exposure 



25 



30 



Figure 10. Cumulative mortality of CF No. 1 female 

 mice irradiated with 160 rad (approximately one-half 

 the LD50/30 dose), and, 33 days later, exposed to a 

 second dose of 340 rad. Compared with age controls 

 irradiated with a single dose of 340 rad fission neutrons. 



comparative half-recovery time after fission-neutron irradiation \v'ould be 

 about 10 days. 



The same split-dose technique has been used in recent experiments in 

 which the time between the two equal doses of fission neutrons has been 

 extended to one or two months. 



In Figure 10 are compared the cumulative mortality data for two groups 

 of mice exposed to fission neutrons. Group A (30 mice) received one 

 exposure of 160 rad, followed 33 days later by a second exposure of 340 rad 

 at the same rate. These should be compared with the mortality data of 

 32 mice irradiated only once with 340 rad. (The second exposures for the 

 two groups were carried out simultaneously in the same irradiation cages.) 

 In each group only one mouse survived the acute period (LD97 30). There is 



232 



