Fast neutron detection with germanium detectors: Unfolding the 692 keV peak response for fission neutron spectra

A new method is proposed for obtaining information on the spectral distribution of fast fission neutrons in the energy range from 0.7 MeV up to about 6 MeV from the shape of the Ge 692 keV inelastic scattering peak response e.g. in the pulse-height distribution measured with a portable Ge detector. In order to reduce photon background events, a lead shielding is put around the active volume of the Ge detector. Peak responses associated with the 692 keV peak are computed by Monte-Carlo simulations including the lead shield. A set of 80 response functions for neutron energies in the aforementioned energy range are used in an unfolding procedure based on least-squares methods for the 692 keV peak response. The procedure was used to determine shape and magnitude of a fission neutron spectrum from a 252Cf source. In a numerical example, the procedure was also applied to unfolding the calculated peak response to spectra of fission neutron sources shielded by carbon, concrete enriched with iron, and a thick lead shield. It is shown that the peak responses for such different spectra can be substantially different and, thus, used for a limited neutron spectrometry.