Prompt fission γ-ray data from spontaneous fission and the mechanism of fission-fragment de-excitation

¹ European Commission, Joint Research Centre, Directorate for Nuclear Safety and Security, Unit G.2 Standards for Nuclear Safety, Security and Safeguards, 2440 Geel, Belgium
² Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia
³ Extreme Light Infrastructure - Nuclear Physics (ELI-NP) / Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN-HH), Bucharest-Magurele 077125, Romania

^a e-mail: stephan.oberstedt@ec.europa.eu
^b The European Commission is gratefully acknowledged for providing a PhD fellowship to one of the authors (A. Gatera).

Published online: 13 September 2017

Abstract

The investigation of prompt γ-ray emission in nuclear fission has a great relevance for the assessment of prompt heat generation in a reactor core and for the better understanding of the de-excitation mechanism of fission fragments. Some years ago experimental data was scarce and available only from a few fission reactions, ^233,235U(n_th, f), ²³⁹Pu(n_th, f), and ²⁵²Cf(sf). Initiated by a high priority data request published by the OECD/NEA a dedicated prompt fission γ-ray measurement program is being conducted at the Joint Research Centre Geel. In recent years we obtained new and accurate prompt fission γ-ray spectrum (PFGS) characteristics (average number of photons per fission, average total energy per fission and mean photon energy) from ²⁵²Cf(sf), ²³⁵U(n_th, f) and ^239,241Pu(n_th, f) within 2% of uncertainty. In order to understand the dependence of prompt fission γ-ray emission on the compound nuclear mass and excitation energy, we started a first measurement campaign on spontaneously fissioning plutonium and curium isotopes. Results on PFGS characteristics from ^240,242Pu(sf) show a dependence on the fragment mass distribution rather than on the average prompt neutron multiplicity, pointing to a more complex competition between prompt fission γ-ray and neutron emission.