Microdosimetric Spectra Measurements of Janus Neutrons

Neutron radiation from the JANUS reactor at Argonne National Laboratory is being used with increasing frequency for major biological experiments. The fast neutron spectrum has a kerma-weighted mean energy of 0.8 MeV and low gamma ray contamination. In 1984 the JANUS fission converter plate of highly enriched uranium was replaced by one made of low enriched uranium. We recorded microdosimetric spectra at several different positions in the high flux irradiation room of JANUS before the change of the converter plate. Each set of measurements consisted of spectra taken at a simulated site diameter of 1 µm in both “attenuator up” and “attenuator down” configurations. At two conventional dosimetry reference positions, two sets of measurements were recorded. At three biological reference positions, measurements simulating several biological irradiation conditions were taken. The dose rate at each position was estimated and compared with dose rates obtained previously by conventional dosimetry. Comparison of the different measurements showed no major change in spectra as a function of position or irradiation condition. First results from similar sets of measurements recorded after the installation of the new converter plate indicate no major change in the spectra.