INJURY AND Rl'.CCn KRV IN XK I rR( ).\-I KRADIA IKD ANIMALS 



'recovery' after fission NKITRON IRRADIATION: PAIRKO EQUAL 

 DOSES WITH VARYING TIME INTERVALS 



Many invcsti,i,^ators have utilized a paired-dose technique to study radiation 

 recovery in animals exposed to X- or y-rays. There are, however, few such 

 fractionation studies following exposure to fission neutrons. 



Mice (female CF No. 1 of 20 to 25 g body weight) have been irradiated 

 with equal paired doses of both «"Co y-rays and fission neutrons in order to 



Single exposure 1 hour . 

 / (361 rad) 



Paired equal doses 

 with Intervals of 



y- 



3 5 days 



-I 



5 days f 



f^ 



;.^7daysy;^^ 



I 1 I 1 



'5^ '10 15 20 25 30 

 Days after neutron irradiation 



Figure 8. Acute 30-day mortality in CF No. 1 

 female mice irradiated with a single exposure of 

 fission neutrons (361 rad in 1 hour), and 

 compared with similar mice irradiated with 

 paired equal doses with 3-5-, 5-, 7-, and 10-day 

 intervals between the paired doses. It is evident 

 that there is recovery after paired exposure to 

 fassion neutrons. 



Study and compare the recovery rates following exposure to these two 



radiations. 



Several experiments have now been carried out which show clearly that 

 there is definite recovery after exposure to fission neutrons, when 30-day 

 mortality of mice is used as a criterion. This type of experiment is illustrated 

 in Figure 8. Five groups of 36 mice were irradiated with neutrons. Group 1 

 received a single dose of 361 rad in 1 hour. Thirty-two of 36, or 89 per cent, 

 of these mice died in the 30-day period following exposure. The remaining 

 four groups of mice were exposed to one-half the dose received by the first 

 group (30-minutc rather than 60-minute exposure), and were then exposed 



230 



