Biological Effectiveness of Neutrons from Hiroshima Bomb Replica: Results of a Collaborative Cytogenetic Study

The effectiveness of neutrons from a facsimile of the Hiroshima bomb was determined cytogenetically. The "Little-Boy" replica (LBR), assembled at Los Alamos as a controlled nuclear reactor for detailed physical dosimetry, was used. Of special interest, the neutron energy characteristics (including lineal energy) measured 0.74 m from the LBR were remarkably similar to those calculated for the 1945 Hiroshima bomb at 1 to 2 km from the hypocenter, as shown in a companion dosimetric paper (Straume, et al., Radiat. Res. 128, 133-142 (1991)). Thus we examine here the effectiveness of neutrons closely resembling those that the A-bomb survivors received at Hiroshima. Chromosome aberration frequencies were determined in human blood lymphocytes exposed in vitro to graded doses of LBR radiation (97% neutrons, 3% γ rays). Vials of blood suspended in air at distances up to 2.10 m from the center of the LBR uranium core received doses ranging from 0.02 to 2.92 Gy. The LBR neutrons (Ē ∼0.2 MeV) produced 1.18 dicentrics and rings per cell per Gy. They were more effective than the higher-energy fission neutrons (Ē ∼1 MeV) commonly used in radiobiology. The maximum RBE (${\rm RBE}_{{\rm M}}$) of LBR neutrons at low doses is estimated to be 60 to 80 compared to 60 Co γ rays and 22 to 30 compared to 250-kVp X rays. These results provide a quantitative measurement of the biological effectiveness of Hiroshima-like neutrons.