Correlations between energy and γ -ray emission in Pu239 ( n , f )

Abstract

We study $\ensuremath{\gamma}$-ray emission following $^{239}\mathrm{Pu}(n,f)$ 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 $\ensuremath{\gamma}$-ray multiplicity increases linearly with incident energy below the second-chance fission threshold with a slope of $0.085\ifmmode\pm\else\textpm\fi{}0.010\phantom{\rule{4pt}{0ex}}{\mathrm{MeV}}^{\ensuremath{-}1}$. This linear trend appears to hold for the average excitation energy of the compound nucleus between $9<\ensuremath{\langle}{E}_{x}\ensuremath{\rangle}<19$ MeV. Most of the multiplicity increase comes from an enhancement around a $\ensuremath{\gamma}$-ray energy of 0.7 MeV, which we interpret as stretched quadrupole $\ensuremath{\gamma}$ rays that indicate an increase in total fission-fragment angular momentum with excitation energy.