Fission fragment yield distribution in the heavy-mass region from the Pu239 ( nth,f ) reaction

The fission fragment yield distribution has been measured in the Pu-239(n(th), f) reaction in the mass region of A = 126 to 150 using the Lohengrin recoil-mass spectrometer. Three independent experimental campaigns were performed, allowing a significant reduction of the uncertainties compared to evaluated nuclear data libraries. The long-standing discrepancy of around 10% for the relative yield of A = 134 reported in JEF-2.2 and JEFF-3.1.1 data libraries is finally solved. Moreover, the measured mass distribution in thermal neutron-induced fission does not show any significant dip around the shell closure (A = 136) as seen in heavy-ion fission data of Pb-208(O-18, f) and U-238(O-18, f) reactions. Lastly, comparisons between our experimental data and the predictions from Monte Carlo codes (GEF and FIFRELIN) are presented and discussed.