Speaker
Description
The evaluation state-of-the-art according to the $^{239}$Pu(n,2n) angular-integrated reaction cross section is well synthesized by Fig.1, enclosed, on which are drawn from one side, experimental data and on the other side, some major evaluated curves. Recent evaluations, by contrast to the old JEF-2.2 data file, remain consistent except near threshold and at energies above the maximum of the excitation function. In the light of the long historical story of the JEF(F) European project according to the evaluation of the $^{239}$Pu$(n,2n)^{238}$Pu reaction cross section and the recent measurement by M\'eot \etal~\cite{meo:21}, this presentation aims to shed another light on this topic by reviewing methodically the impact of the level density model for the $^{239}$Pu residual nucleus on the final shape of the left-tail of the excitation function. To achieve this goal, the AVXSF-LNG computer program~\cite{bou:13} has been upgraded to model second-chance reactions using its decay-probability module~\cite{bou:19,bou:20} and, then chained to the TALYS-ECIS06 nuclear reaction system of codes~\cite{kon:12}. This presentation will come back on the question of the most relevant type of level density for second chance preequilibrium neutron emission calculation. According to TALYS, the spin-dependent population calculated after compound nucleus emission works well with the exciton model whereas the MSD-MSC quantum mechanical preequilibrium model is more adequately represented by a spin distribution based on particle-hole state densities. As far as a spectrum simulation of a few number of individual excitations is desired, the combinatorial Quasi-Particle-Vibrational-Rotational Level Density method~\cite{bou:13} implemented in the AVXSF-LNG computer program is well suited to reach that goal. Several residual nucleus level densities have been tested in this work using the AVXSF-LNG code features, including total level density and the level density obtained after compound nucleus emission. This presentation will also include the testing of the Quasi-particle Random Phase Approximation-based result of Ref.~\cite{dup:17}. Finally, the proportion of 'preequilibrium' upon the model of residual nucleus level density, will be quantified against the well-known compound nucleus contribution building then the total (n,2n) cross section.
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\bibitem{meo:21} V.~M\'eot, O.~Roig, B.~Laurent, P.~Morel, J.~Aupiais, O.~Delaune, G.~Haouat and O. Bouland, Phys. Rev C {\bf 103}, 054609 (2021).
%239Pu(n,2n) 238Pu cross section measurement using a recoil method
\bibitem{bou:13} O.~Bouland, J.E.~Lynn and P.~Talou, Phys. Rev. C \textbf{88} 054612 (2013).
%{\it $\mathcal{R}$-matrix analysis and prediction of low-energy neutron-induced fission cross sections for a range of Pu isotopes}
\bibitem{bou:19} O.~Bouland, %{\it Reexamining fission-probability data using $\mathcal{R}$-matrix Monte Carlo simulations: Beyond the surrogate-reaction method},
Phys. Rev. C {\bf 100}, 064611 (2019).
\bibitem{bou:20} O.~Bouland and G.~Nogu`ere, %{\it Reconciling surrogate-reaction probabilities and neutron-induced cross sections},
Phys. Rev. C {\bf 102}, 054608 (2020).
\bibitem{kon:12} A.J.~Koning and D.~Rochman, Nuclear Data Sheets \textbf{113}, Issue 12, Pages 2841-2934 (2012).
%Modern Nuclear Data Evaluation with the TALYS Code System
\bibitem{dup:17} M.~Dupuis, S.~Hilaire, S.~P\'eru, E.~Bauge, M.~Kerveno, P.~Dessagne, and G.~Henning, EPJ Web of Conferences 146, 12002 (2017).