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



to a second 30-minute irradiation after a time interval of 3-5, 5, 7, or 10 

 days, respectively. After these time intervals, the 30-day mortality figures 

 were 36 per cent, 14 per cent, 8 per cent, and 3 per cent as compared to 

 approximately 90 per cent when the single dose was administered. There is, 

 therefore, definite recovery following fractionation of the fission-neutron 



I/) 

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1o 



c 

 o 



o 



Q. 



•D 

 0) 



J> 20 



c 



ID 



10 



X— — « Fission neutrons 



60, 



Co y-rays 



1 2 3 A 5 6 7 8 9 10 11 12 13 14 15 

 Days between paired doses 



Figure 9. Comparative recovery rates in mice exposed to 

 equal paired doses of ^''Co y-rays and of fission neutrons, 

 with intervals between the exposures varying from 1 to 

 12 days. The graph illustrates the unrecovered portion 

 of the first exposure (%) plotted against the time between 

 the paired doses (semi-log plot). 



dose. It would also appear from the data that the amount of recovery was 

 related to the time interval between the equal paired doses. 



A comparison of such paired-dose experiments following both ^•'Co y-rays 

 and fission neutrons has been made. Several interesting facts are evident: 



{]) The relationship between the lethal dose required to kill 50 per cent 

 of the mice in the 30-day period (LDgogp), and the time between doses, at 

 least in the range of 1 to 10 day intervals, seems to be a linear one on a 

 log-log plot, whether the radiation is fission neutrons or y-rays from ®"Co. 



(2) There is evident recovery after fractionation of either dose of radiation. 



(3) The recovery rate, as judged by the slopes of the lines, does not 

 appear to be identical. 



This latter point is emphasized in Figure 9 in which the residual or 

 ' unrecovered ' portion of the first dose (in percentage) is plotted against the 

 time interval in days between equal paired doses. 



231 



