Updating GEANIE $^{239}$Pu prompt $γ$-ray experimental data using modern Hauser-Feshbach fission fragment decay model

TL;DR

This work revisits the historic GEANIE data for neutron-induced reactions on $^{239}$Pu by leveraging modern Hauser-Feshbach approaches: HF3D BeoH for prompt fission-fragment gamma emissions and CoH3 for (n,n'), (n,2n), (n,3n), and (n,4n) gamma production. By estimating and subtracting the fission and other background gamma contributions, the authors obtain corrected partial gamma-ray cross sections that better agree with statistical-model predictions and use them to infer the total inelastic, (n,2n), and (n,3n) cross sections. The results show improved consistency with CoH3 below roughly 10 MeV for several channels, though some discrepancies—especially in (n,3n)—remain due to potential unaccounted backgrounds or branching uncertainties. The methodology demonstrates how prompt gamma measurements can salvage and refine actinide cross sections when fission backgrounds are substantial, informing future experiments and evaluations.

Abstract

We calculate fission $γ$ rays for neutron-induced reactions on $^{239}$Pu with the Hauser-Feshbach fission fragment decay model. By applying the calculated fission $γ$ rays as a background contribution, the historical $^{239}$Pu(n,$x$n$γ$) reaction cross section data measured by the GEANIE (GErmanium Array for Neutron Induced Excitations) spectrometer are corrected. The correction also includes other (n,$x$n) reactions that have very similar energies to the $γ$ lines reported by GEANIE. In many cases, the original GEANIE data are strongly reduced and they become much closer to the statistical Hauser-Feshbach model predictions. The total inelastic scattering, (n,2n), and (n,3n) cross sections are inferred based on the corrected GEANIE data, and compared with available experimental data as well as the statistical model calculations. Expected $γ$-ray energy spectra for neutron-induced measurements on $^{239}$Pu are also discussed.

Figures (20)

• Figure 1: Calculated $\gamma$-ray production cross sections for the neutron-induced reaction on $^{239}$Pu at 19.3 MeV as function of the $\gamma$ energy. The symbols represent $\gamma$ lines from the fission fragments. The vertical lines are from the (n,$x$n) reactions, where $x=1$, 2, and 3.
• Figure 2: Calculated $\gamma$-ray spectra from the fission fragments (dot-dashed curve), and (n,$x$n$\gamma$) reactions (solid) for the neutron incident energy of 11.4 MeV. The total spectrum is shown by the dashed curve. This figure simulates Figs. 4a in Ref. UCRL-ID-140308 before background subtraction, which is shown by the histogram.
• Figure 3: Calculated $\gamma$-ray spectra from the fission fragments and 157.4-keV (n,2n$\gamma$) reaction for the neutron incident energy of 11.4 MeV.
• Figure 4: Calculated 226.4-keV $\gamma$-ray production cross section by the (n,n'$\gamma$) reaction, compared with original and corrected GEANIE data.
• Figure 5: Calculated 154.7-keV $\gamma$-ray production cross section by the (n,n'$\gamma$) reaction, compared with original and corrected GEANIE data.