This series of experiments utilized the depletion of spermatogonia in mice, as evidenced by the reduction of (subsequent) resting primary spermatocytes, to assay the biological effectiveness of radiations differing in quality and dose rate. Both 250-kVp x-rays and60 Co gamma rays yielded 50% effective doses near 40 rads, and no dose-rate effect for the former could be demonstrated in the range of 0.2 to 100 rads/min. Monoenergetic neutrons had ED50 values of 3.7 rads at 0.43 MeV and 15 rads at 14 MeV. Neutrons of intermediate energies yielded${\rm ED}_{50}\text{'}{\rm s}$ between these two values. No influence of dose rate at the 50% level of effect could be demonstrated with 0.62-MeV neutrons in the range of 0.015 to 1.5 rads/min or with 14-MeV neutrons in the range of 0.15 to 15.0 rads/min. A comparison of minimal gross sensitive volumes (gsv), as obtained from the inactivation constants, with measurements of spermatogonia in fresh preparations showed the gsv to be similar to the spermatogonia cell volume.

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