Evaluation of high-energy delayed gamma-ray spectra dependence on interrogation timing patterns

The Integrated Support Center for Nuclear Nonproliferation and Nuclear Security of the Japan Atomic Energy Agency is developing delayed gamma-ray spectroscopy (DGS) for nuclear safeguards. Intended to supplement the present verification methods of spent nuclear fuel, we are evaluating the DGS capability to quantify the U and Pu fissile composition using this non-destructive assay technique. Toward this goal, we perform experiments using the PUNITA instrument in collaboration with the European Commission Joint Research Centre in Ispra, Italy. Our studies presented here focus on evaluating the gamma-ray spectral dependence on the fission product decay times. Specifically, we investigated multiple time patterns to evaluate the observable gamma rays useful to quantify the ²³⁹Pu and ²³⁵U contributions. Of the time patterns tested, spectral distinction is best with a 60-s irradiation followed by a 60-s gamma-ray measurement for a comparable one-hour total interrogation time.