Reversed dose-rate effect and RBE of 252-californium radiation in the induction of 6-thioguanine-resistant mutations in mouse L5178Y cells.

The effects of californium-252 radiation (average neutron energy E = 2.13 MeV) were investigated using mouse leukemia L5178Y cells. No dose-rate effect was detected for cell killing, but a 'reversed' dose-rate effect was observed for mutation induction. The frequency of 6-thioguanine-resistant mutations increased linearly up to 100 cGy (1 Gy = 100 rad), then began to level off at a dose rate of 1.2 cGy/min, while it increased continuously up to 200 cGy at a reduced dose rate of 0.16 cGy/min. Compared with results obtained using 60Co gamma-rays, the ratio of the initial slope of each dose-response curve was 4-5 for cell killing, and more than 11 for mutagenesis. Since one-third of 252Cf radiation consists of gamma-rays, the relative biological effectiveness (RBE) of 252Cf neutrons would be even greater, 16 or more, for mutation induction in the present assay.