Far-field fast-neutron energy spectra from an unshielded fission reactor

摘要

To a large extent, radiation exposure, radiation measurements, and radiation protection factors depend upon the assignment of doses to the survivors of the nuclear bombings of Hiroshima and Nagasaki. For neutrons, the problem of correlating dose with fluence inevitably leads to questions about air-over-ground transport calculations, which have been tested primarily against benchmark measurements using the reactor at the Army Pulse Radiation Facility at Aberdeen Proving Ground, Maryland. At the ranges of 1000-2000 m that are most relevant for the Hiroshima survivors, the agreement between the calculated and measured integral dose values is generally within 10%, but comparisons of the differential values differ by as much as 40%. Using a new type of neutron spectrometer, we report measured neutron energy spectra from 0.5 to 10 MeV at distances of 1080 m and 1620 m, with results that generally confirm those of other experiments. Based on studies of transmission (open quotes)windows(close quotes) through nitrogen/oxygen mixtures and the effect of scattering from the ground near the detector, calculations that take into account the detector surroundings and use ENDF/B-VI cross sections are able to obtain agreement within 20% at almost all energies.