# Low-lying level structure of $^{56}$Cu and its implications on the rp process

## TL;DR

This study investigates the low-energy structure of $^{56}$Cu using advanced gamma-ray spectroscopy, providing new reaction rates that influence the understanding of the rp-process in x-ray bursts.

## Contribution

It presents the first experimentally-constrained reaction rate for $^{55}$Ni(p,$$)$^{56}$Cu based on new level structure data, impacting rp-process modeling.

## Key findings

- The reaction rate for $^{55}$Ni(p,$$)$^{56}$Cu has been experimentally constrained.
- The rp-process can bypass the $^{56}$Ni waiting point with a branching of up to 40%.
- Additional nuclear physics uncertainties remain to be addressed.

## Abstract

The low-lying energy levels of proton-rich $^{56}$Cu have been extracted using in-beam $\gamma$-ray spectroscopy with the state-of-the-art $\gamma$-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in $^{56}$Cu serve as resonances in the $^{55}$Ni(p,$\gamma$)$^{56}$Cu reaction, which is a part of the rp-process in type I x-ray bursts. To resolve existing ambiguities in the reaction Q-value, a more localized IMME mass fit is used resulting in $Q=639\pm82$~keV. We derive the first experimentally-constrained thermonuclear reaction rate for $^{55}$Ni(p,$\gamma$)$^{56}$Cu. We find that, with this new rate, the rp-process may bypass the $^{56}$Ni waiting point via the $^{55}$Ni(p,$\gamma$) reaction for typical x-ray burst conditions with a branching of up to $\sim$40$\%$. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the $^{56}$Ni region.