Effects of Subthreshold Fission on Neutron Dose Measurements Made With 237Np Detectors

Abstract

Although the fission cross section of 237Np has a "threshold" at about 200 keV, it is still significant at lower energies. To facilitate corrections of 237Np dosimeter readings for this subthreshold response, recently measured cross sections between 1 and lo5 eV have been approximated by an analytical function of energy. This was used to calculate the contribution of subthreshold neutrons to the response of Np dosimeter for measurements of fission neutrons transmitted through various materials. For most of the spectra, the effect was less than 3%, but for some it was several times larger. The fission resonance integral, from 0.5 eV to 100 keV. is 280 mb.