Correlations between energy and $\gamma$-ray emission in $^{239}\mathrm{Pu}(n,\mathrm{f})$

TL;DR

This study investigates gamma-ray emissions following neutron-induced fission of plutonium-239, revealing a linear increase in gamma-ray multiplicity with incident energy and a correlation with fission fragment angular momentum.

Contribution

First experimental evidence linking total angular momentum in fission to excitation energy, with detailed gamma-ray multiplicity behavior across a range of neutron energies.

Findings

Gamma-ray multiplicity increases linearly with incident neutron energy.

Enhancement in gamma-ray emission around 0.7 MeV indicates increased angular momentum.

Linear trend in gamma-ray multiplicity holds for excitation energies between 9 and 19 MeV.

Abstract

We study $\gamma$-ray emission following $^{239}\mathrm{Pu}(n,\mathrm{f})$ over an incident neutron energy range of $2 < E_i < 40$ MeV. We present the first experimental evidence for positive correlations between the total angular momentum generated in fission and the excitation energy of the compound nucleus prior to fission. The $\gamma$-ray multiplicity increases linearly with incident energy below the 2\textsuperscript{nd}-chance fission threshold with a slope of $0.085 \pm 0.010$ MeV$^{-1}$. This linear trend appears to hold for the average excitation energy of the compound nucleus between $9 < \langle E_x \rangle < 19$ MeV. Most of the multiplicity increase comes from an enhancement around a $\gamma$-ray energy of 0.7 MeV, which we interpret as stretched quadrupole $\gamma$ rays that indicate an increase in total fission-fragment angular momentum with excitation energy.