The intestinal microcolony assay technique, for measuring damage to the progenitor cell population of mouse jejunal epithelium, has been used to demonstrate the following: (a) Single doses of collimated 14-MeV D-T neutrons (10-30 rad min-1) are more effective than single doses of 300-kV X-rays or Cs-137 γ-rays (30-650 rad min-1) by a factor of 1.9-2.3 for a reduction in crypt numbers of 50-80%. (b) For mice irradiated at 10 cm deep in a 30-cm-deep water phantom, the above RBE values are reduced by about 9%. The same reduction is applicable for single doses and for four daily dose fractions. (c) With daily fractionated irradiations (up to 5), the iso-effect curves (for 50% or 80% crypt number depopulation) are linear on a log/log plot, with slope exponents of 0.57-0.58 (γ-rays) and 0.32-0.34 (neutrons). Estimates of the RBE values for extended fractionated dose regimens are 3.3 (10 daily fractions) and 3.6 (15 daily fractions).

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