Life shortening in mice exposed to fission neutrons and γ rays. II. Duration-of-life and long-term fractionated exposures

Data are presented on the life shortening of B6CF1 mice given weekly exposures to 0.85-MeV fission neutrons (0.67, 1.67, 2.67 rad per weekly fraction) or 60Co γ rays (7, 17.4, 31.9 rad per weekly fraction) either for 60 exposures or for the duration of their lives. The life shortening produced by either the duration-of-life or the fractionated exposures was virtually identical, so the two series were treated as a single experiment. The relative biological effectiveness (RBE) for neutrons increased from 12 at 2.67 rad per weekly fraction to 21 at 0.67 rad per weekly fraction; over the dose ranges studied, the neutron RBE varied inversely with the -0.4 power of the dose. It is predicted that at 0.05 rad of neutrons per week, the RBE will approach 60 if weekly exposures to γ radiation are chosen as the reference; if continuous γ radiation is used as the reference, the RBE may be approximately twice as large