Use of delayed gamma spectra for detection of actinides (U,Pu) by photofission

The development of non-destructive methods to inspect nuclear waste containers is important for radioactive waste management and non proliferation purposes. Among methods using nuclear radiation as a probe, instrumental photon activation analysis (IPAA) seems to be a promising way forward. We have previously developed an IPAA method to determine the mass of actinides in radioactive waste sealed in concrete by counting the delayed neutron emission after photofission. This method does not determine the nature of the actinides detected. Such additional useful information may, however, be derived from analysis of the delayed gamma emission spectrum. The main topic of this paper is to show how analysis of the delayed gamma spectra may overcome some of the limitations of delayed neutron analysis. Target samples of 93% and 25% enriched 235U and Pu were irradiated with bremsstrahlung gamma-rays produced by 15-MeV electrons from a linear accelerator. The gamma-rays spectra for each of the two uranium isotopes studied reveals a distinctive intensity distribution which is the consequence of modification of light-wing fission products distribution.