Neutron Leakage from 'COMET' - A Duplicate Little-Boy Device.

Abstract : Fast neutron spectra between 600 keV and 10 MeV, directed outwards from the surface of the 'COMET' assembly, were measured using an NE-213 fast-neutron spectrometer at the Los Alamos National Laboratory. The COMET experiment, consisting of a duplicate Little-Boy device suitably modified for operation in the delayed-critical regime, was to provide improved energy and angular distributions of leakage spectra in support of efforts to re-evaluate ground doses from the Hiroshima bombing. Measurements were obtained at seven polar angles (from 0 to 135 degrees) and at two radii from the centre of the active volume (75 and 200 centimeters). These measurements were compared to two calculations available at the time of writing; one an earlier (1976) one-dimensional estimation and another more recent (1982) two-dimensional calculation, both based on Monte-Carlo techniques. Differences in high energy neutron leakage are apparent and probably due to dynamical considerations, as the theoretical calculations simulated the disassembling weapon itself, rather than the static experiments described herein. (Author)