HOWARD H. VOGEL, JR., DONN L. JORDAN AND SAMUEL LESHER 



no evidence from this experiment that there is any residual or unrecovered 

 portion left from the first 160 rad of fission neutrons after the interval of 

 33 days, as measured by this technique. 



In Figure 11 are summarized the mortality data from a similar exposure 

 to split doses of fission neutrons, carried out at the CP-5 research reactor, 

 with an interval of two months between exposures. It is clear from these data 

 that mice exposed to 100 rad or 185 rad 60 days prior to a second exposure 

 of 300 rad of fission neutrons, showed no increase in 30-day mortality when 



.100 



90 



80 



70- 



5 60 



I.. 



o 



E 

 . 50 



_> 



13 



3 AO 



£ 



^30 



20 



10 



8 12 16 20 lU 



Days after irradiation 



28 



Figure 11. Cumulative mortality of GF No. 1 female 

 mice irradiated with 300 rad fission neutrons, 2 months 

 after a preliminary exposure of 100 rad or of 185 rad 

 of neutrons. Gompare with age control group which 

 were irradiated only once with 300 rad. 



compared with a group of age controls simultaneously exposed to a single 

 dose of 300 rad. There is even an indication, since the mortality in both 

 split-dose groups is lower than that in the age controls, that the first dose of 

 neutrons might be slightly 'beneficial' as measured by 30-day mortality. 

 At any rate, we may conclude that 60 days after a first neutron dose (from 

 30 per cent to 60 per cent of the LDgQ g^, single, whole-body exposure), there 

 seems to be no residuum as measured by the 30-day acute mortality criterion. 

 In this paper, we have limited our consideration of recovery data to 

 neutron-irradiated mice. However, parallel experiments have recently been 

 reported describing recovery in the four-day chick embryo, when irradiated 

 with paired doses of fission neutrons with only two to three hours between 

 exposures^". Daniels and Vogel^^ have also reported complete recovery in 



233 



