Year
2003
Abstract
The Combined Thermal Epithermal Neutron (CTEN) assay system is employed at the Los Alamos National Laboratory to assay standard 208-L containers of plutonium bearing waste using either (or both) an active or passive neutron assay. CTEN was designed to employ both cadmium shielded and unshielded (bare) 3He neutron detectors in performing a passive assay. Passive assays of nuclear material make use of these detectors to count high-energy prompt neutron events generated by spontaneous neutron emissions, alpha capture neutron emissions (a, n), and induced neutron emissions. The cadmium shielding selectively passes high-energy prompt neutrons to the 3He detector while blocking thermalized 14 MeV neutrons that are used in an active assay to induce fission. The original intent of the bare detectors was to improve the efficiency of a passive assay since all neutrons are counted, but this also increases noise. However, bare detector data was recently omitted from CTEN assay measurements due to intermittent problems with the bare detectors, leaving only the shielded detectors available for the assay. Analysis of background measurements was made over the period Sep 2000 through Sep 2002. The unshielded (bare) 3He neutron detectors in the CTEN assay instrument improve the lower limit of detection (LLD) of the instrument by approximately 12%, which is significantly lower than the theoretical 69% improvement. The case for using shielded detectors only is that they are less susceptible to background variation, such as cosmic radiation.