Using low resolution gamma detectors to detect and differentiate 239Pu and 235U fissions

When 239Pu and 235U undergo thermal neutron-induced fission, both produce significant numbers of beta-delayed gamma rays with energies in the several megaelectron volt range. Experiments using high energy-resolution germanium detectors have shown that it is possible to distinguish the fission of 239Pu from that of 235U. It is desirable to detect the presence of 235U or 239Pu using detectors that are less expensive and more rugged than high purity germanium detectors. To this end we demonstrate how differences in the energy spectrum and decay rates of the beta-delayed gamma rays can be used to identify 239Pu and 235U using low resolution plastic and liquid scintillator detectors. Experimental data are used to identify differences in the spectra and also to test the identification algorithms. Results to date are very promising.