Correlating the fissile mass of standard uranium samples with delayed gamma rays from fission products

Abstract

The Japan Atomic Energy Agency and the European Commission Joint Research Centre are collaborating to develop delayed gamma-ray spectroscopy (DGS) for nuclear materials for safeguards verification in reprocessing plants. In this paper, we describe DGS interrogation using the Pulsed Neutron Interrogation Test Assembly with standard samples of different 235U enrichments. By analyzing gamma-ray spectra, we reveal a linear correlation between the sample mass and both the total counts above 3.3 MeV and the peak counts of specific high-energy gamma-ray. We were able to observe, qualify and quantify specific gamma rays peak down to the depleted uranium (0.5 g 235U) mass sample. Based on this, we demonstrate that our technique is able to estimate the total fissile mass with a statistical uncertainty <2% when taking into account self-shielding and gamma self-absorption corrections. Using integrated counts above 3.3 MeV we were able to reduce the mass-dependent bias for the higher enrichments (∼3 to 4%) to <4%.