Mice (C57BL and C3H) and Sprague-Dawley rats were exposed to 250 kVp x-rays or 14 MeV neutrons, and mortality data were collected for 30 days thereafter. The whole-body, bilateral x-irradiations over the range of 378-918 rads (midline tissue dose) were delivered at approximately 21 rads/minute. The whole-body rotational neutron irradiations utilized midline tissue doses in the range from 282 to 707 rads. C57BL mice and Sprague-Dawley rats were exposed at a neutron dose rate which varied from 15 to 35 rads/minute (average ∼20 rads/minute). C3H mice were exposed at 15-50 (average ∼20 rads/minute) or 3-5 rads/minute to investigate the possibility of dose-rate effects. Mortality or survival time differences were not observed between sexes, and reported values represent combined groupings. The${\rm LD}_{50/30}$ values for x-rays and 14 MeV neutrons, respectively, were: C57BL mice, 680 and 432 rads; C3H mice, 704 and 537 (3-5 rads/minute) or 480 (15-50 rads/minute); Sprague-Dawley rats, 810 and 494 rads. The relative biological effectivenesses (RBE) for 14 MeV neutrons using 30-day mortality as the end point for comparison and 250 kVp x-rays as the reference radiation were 1.6, 1.5, and 1.6 for C57BL mice, C3H mice, and Sprague-Dawley rats, respectively. A dose-rate effect was evident since the${\rm LD}_{50/30}$ value for C3H mice exposed at 15-50 rads/minute was significantly lower (480 rads) than that of mice exposed at 3-5 rads/minute (537 rads). Of the strains studied only the mean survival time of C3H mice was significantly less when exposed to 14 MeV neutrons at 15-50 rads/minute than when exposed to x-rays.

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