Enhanced low-energy $\gamma$-decay strength of $^{70}$Ni and its robustness within the shell model

TL;DR

This study measures and analyzes the enhanced low-energy gamma-decay strength in neutron-rich $^{70}$Ni, revealing its robustness and implications for nuclear structure and astrophysical processes.

Contribution

It provides the first measurement of low-energy gamma-decay strength in a very neutron-rich nucleus and demonstrates its robustness across different theoretical models and isotopes.

Findings

Significant enhancement in gamma-decay strength below 3 MeV in $^{70}$Ni.

The low-energy gamma strength is likely of M1 character, consistent across models.

Predicted to be present in other neutron-rich Ni isotopes, affecting nucleosynthesis models.

Abstract

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\gamma$-emission probability at very low $\gamma$ energies. In this work, we present measurements of the $\gamma$-decay strength of $^{70}$Ni over the wide range $1.3 \leq E_{\gamma} \leq 8 $ MeV. A significant enhancement is found in the $\gamma$-decay strength for transitions with $E_\gamma < 3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above $E_\gamma \simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\gamma$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.