First evidence of enhanced low-energy $γ$-ray strength from thermal neutron capture data

The $γ$-ray strength function, or average reduced $γ$-ray transition probability, is a fundamental input in the calculation of $(n,γ)$ cross sections used to simulate the nucleosynthesis of elements heavier than Fe. Since 2004, an enhanced probability of $γ$-decay with $γ$-ray energies below $\approx2 - 4$ MeV has been measured in reaction data for numerous nuclei. This has been observed as an increase in the $γ$-ray strength with decreasing $γ$-ray energy, often referred to as the low-energy enhancement or \textit{upbend} in the $γ$-ray strength. Nevertheless, the available data confirming this enhancement corresponded solely to charged-particle included reactions and no low-energy enhancement had yet been confirmed from neutron-induced reaction measurements. In this work, we present the first evidence of low-energy $γ$-ray strength enhancement from neutron-capture reaction data. Gamma-ray spectra following thermal neutron capture on $^{58,60}$Ni have been used to determine the strength for primary and secondary $γ$-rays in $^{59,61}$Ni, showing an enhancement for $γ$-ray energies below $\approx 3$ MeV and $\approx 2$ MeV for $^{59,61}$Ni, respectively. For the first time, this enhancement is observed down to $γ$-ray energies of $\approx0.2$ MeV. Further, available spin-parity assignments have been used to obtain the multipolarity and electromagnetic character of these transitions, showing that this low-energy enhancement is dominated by $M1$ and $E2$ strength, with $E1$ strength also exceeding Standard Lorentzian Model predictions. Finally, large-basis shell-model calculations have been performed, also predicting a strong $M1$ enhancement at low $γ$-ray energies.