Neutron induced recoil protons of restricted energy and range and biological effectiveness

Low energy neutrons (<2 MeV), those of principal concern in radiation protection, principally initiate recoil protons in biological tissues. The recoil protons from monoenergetic neutrons form rectangular distributions with energy. Monoenergetic neutrons of different energies (<2 MeV) will then produce overlapping recoil proton spectra. By overlapping the effects of individual deposition events, determined microdosimetrically for cell nuclear dimensions, from such neutron beams the biological effectiveness of recoil protons within defined energy and range bounds can be determined. Here chromosomal aberrations per cell have been quantified following irradiation of Vicia faba cells with monoenergetic neutrons of 230, 320, 430, and 1,910 keV. Aberration frequencies from cells from part of the cell cycle, thereby limiting nuclear dimensions, were linearly related to dose and to the frequency of proton recoils per nucleus. The 320 keV neutrons were the most biologically effective per unit absorbed dose and 430 keV neutrons most effective per recoil proton, with 21% of recoils inducing aberrations. After extraction of effectiveness per proton recoil within each energy and range bounds (0-230, 230-320, 320-430, and 430-1,910 keV), it was concluded that recoil protons with energies of about 200-300 keV, traveling 2.5-4 microm and depositing energy at about 80 keV micrometer(-1), are more efficient at aberration induction than those recoil protons of lesser range though near equivalent LET and those of greater range through lesser LET. This approach allows for assessment of the biological effectiveness of individual energy deposition events from low energy neutrons, the lowest dose a cell can receive, and provides an alternative to considerations of relative biological effectiveness.