The fission fragment yield distribution has been measured in the ${}^{239}\mathrm{Pu}\left(n_{\mathrm{th}},f\right)$ 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 ${}^{208}\mathrm{Pb}$(${}^{18}\mathrm{O}$, $f$) and ${}^{238}\mathrm{U}$(${}^{18}\mathrm{O}$, $f$) reactions. Lastly, comparisons between our experimental data and the predictions from Monte Carlo codes (gef and fifrelin) are presented and discussed.

• Received 13 February 2017
• Revised 17 April 2017

DOI:https://doi.org/10.1103/PhysRevC.96.014608