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

Stephan Oberstedt,Aleksandar Dragic,Franz-Josef Hambsch,Alf Göök,Angelique Gatera,Andreas Oberstedt

doi:10.1051/epjconf/201714604060

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, 233,235 U(nth , f), 239 Pu(nth , f), and 252 Cf(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 252 Cf(sf), 235 U(nth , f) and 239,241 Pu(nth , 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 240,242 Pu(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.

Highlights

Some years ago new interest was shown in the measurement of prompt fission γ -ray spectra (PFGS)
This was motivated by requests for new precise values especially for γ -ray multiplicities and average photon energy release per fission in the thermal-neutron induced fission on 235U [1] and 239Pu [2]
The prompt fission γ -ray measurements on 240Pu and 242Pu formed part of a series of experiments, which originally aimed at the precise measurement of their neutron-induced fission cross-sections, in response to a high-priority request published through the OECD/NEA [12, 13]

Summary

Introduction

Some years ago new interest was shown in the measurement of prompt fission γ -ray spectra (PFGS). This was motivated by requests for new precise values especially for γ -ray multiplicities and average photon energy release per fission in the thermal-neutron induced fission on 235U [1] and 239Pu [2]. Improvements of nuclear data have become possible due to advances in scintillator materials, as used e.g. in lanthanide halide detectors They offer a superior combination of intrinsic peak efficiency, energy and timing resolution, as already demonstrated in a number of recent experiments on 252Cf(sf) [3,4,5], 235U(nth, f) [4, 6] and 241Pu(nth, f) [4, 7]. We obtained first results on PFGS characteristics from the spontaneous fission of 240Pu and 242Pu [8]

Experiment and data treatment

Results and discussion

Conclusion