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 dist ributions with energy. Monoenergetic neutrons of different energies (< 2 MeV) will then produce overlapping recoil proton spectra. By overla pping the effects of individual deposition events, determined microdos imetrically for cell nuclear dimensions, from such neutron beams the b iological effectiveness of recoil protons within defined energy and ra nge bounds can be determined. Here chromosomal aberrations per cell ha ve been quantified following irradiation of Vicia faba cells with mono energetic neutrons of 230, 320, 430, and 1,910 keV. Aberration frequen cies from cells from part of the cell cycle, thereby limiting nuclear dimensions, were linearly related to dose and to the frequency of prot on recoils per nucleus. The 320 keV neutrons were the most biologicall y effective per unit absorbed dose and 430 keV neutrons most effective per recoil proton, with 21% of recoils inducing aberrations. After ex traction of effectiveness per proton recoil within each energy and ran ge bounds (0-230, 230-320, 320-430, and 430-1,910 keV), it was conclud ed that recoil protons with energies of about 200-300 keV, traveling 2 .5-4 mu m and depositing energy at about 80 keV mu m(-1), are more eff icient at aberration induction than those recoil protons of lesser ran ge though near equivalent LET and those of greater range through lesse r LET. This approach allows for assessment of the biological effective ness of individual energy deposition events from low energy neutrons, the lowest dose a cell can receive, and provides an alternative to con siderations of relative biological effectiveness.