= wm CONTROL| x i = 7 RANGE o = a - t— 20 = ;—— — = “-—--~ _ = s = * <x bu = O 0 _t | 40 ! —_L 80 ! ] 120 | 160 DURATION OF EXPOSURE, WEEKS Figure XII. Mean survival times of female CBA mice after chronte terminated and duratton-of-life exposures to gamma rays and fast neutrons. The nominal weekly doses are: gamma rays, @ 350 R, o 210 R, 0110 R, fast neutrons, A 16 rad, ¥ 2 rad. V 16 R; In duration-of-life experiments the mean survival time equals the mean length of exposure and therefore all potnts for such exposures must lie on the dtagonal slope 1, as shown. Vertical bars show the range of mean survival time tn vartous groups of control mice. Data from Mole and Thomas [M7]. 176. In the series by Bustad et al. [B17] hybrid (C57BLx101) mice were ex- posed for 8 hours daily from the age of 6 to the age of 58 weeks (that is for one year) to 0.1 R/hour or 0.2 R/hour of 604, gamma radiation. The cumulated total doses were 290 and 480 R, respectively, after which the animals were taken out of the radiation field and followed for the rest of their life. Al- though the treatment did produce some small differences in the average pattern of growth and longevity, in most instances these differences were found to be small relative to the variability of the normal samples. 177. Grahn, Fry and Lea [G5] also discussed the effect of protraction on mortality, showing that the risk of death from all and from specific causes is quite different if compared to acute exposure. Their data on LAF1 mice show that leukaemia death-rate is reduced by a factor of 5 or more daily exposure levels below 20-30 R/day. Data on all causes of death other than leukaemia show