The Survival of Colony-forming Units in Mouse Bone-marrow afterin VivoIrradiation with D–T Neutrons, X- and Gamma-radiation

Abstract

In collaborative experiments the CFU survival in mouse bone-marrow has been investigated after in vivo irradiation with D–T neutrons, 300 kV x-rays and ⁶⁰Co and ¹³⁷Cs gamma-rays. All the survival curves for x- and gamma-radiation showed the same extrapolation number (2·47 ± 0·28), whereas D₀ values of 72·2 ± 1·7 rads and 90·7 ± 2·4 rads, respectively, were found for the two types of radiation. With D–T neutrons, the Manchester group found a D₀ value of 62·5 ± 2·2 rads; the Rijswijk group found a D₀ value of 78·3 ± 2·1 rads. A common extrapolation number of 1·34 ± 0·16 could be calculated for the neutron survival curves. Owing to the difference in extrapolation numbers for x-rays and neutrons, the RBE depends on the level of survival. From the combined results of both groups a mean RBE of 1·1 can be derived for low surviving fractions. The 25 per cent difference in the effectiveness of D–T neutrons, as measured by the two groups, was not due to dose-rate nor to the strain of mice, and could only in part be attributed to the 8 per cent difference in dosimetry revealed by intercomparative measurements. It was probably due mainly to a difference in quality of the two neutron beams resulting from the collimation of the Manchester beam with consequent degradation of neutron energy.