Calibration of neutron sensitive spherical devices with a D2O-moderated 252Cf source at close distances

The aim of the present work was to investigate the dependence of the response of spherical devices at much smaller center-to-center separation distances Do, and to try to explain any departures from the inverse-square law. The response of spherical devices due to a physically small radionuclide source as a function of the separation distance, at both small and large distances, is well understood. Departures from the inverse square-law at close distances can be explained in terms of a geometric correction that takes into account the finite size of the detector and the divergence of the incident neutron fluence. If the response of a spherical device could be understood at such close distances, for a physically large source such as the D2O-moderated 252Cf source, then calibrations could be carried out over the full operational range of typical devices using neutron sources with relatively low emission rates. This would reduce the initial cost of the source to the calibration laboratory and ease the problems associated with its handling