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

The investigation of prompt gamma-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, U-233,U-235(n(th), f), Pu-239(n(th), f), and Cf-252(sf). Initiated by a high priority data request published by the OECD/NEA a dedicated prompt fission gamma-ray measurement program is being conducted at the Joint Research Centre Geel. In recent years we obtained new and accurate prompt fission gamma-ray spectrum (PFGS) characteristics (average number of photons per fission, average total energy per fission and mean photon energy) from Cf-252(sf), U-235(n(th), f) and Pu-239,Pu-241(n(th), f) within 2% of uncertainty. In order to understand the dependence of prompt fission gamma-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 Pu-240,Pu-242(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 gamma-ray and neutron emission.