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

Abstract

The low-lying energy levels of proton-rich ${}^{56}$Cu have been extracted using in-beam ${\hat{I}}^3$-ray spectroscopy with the state-of-the-art ${\hat{I}}^3$-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,${\hat{I}}^3$)${}^{56}$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 $Q=639\pm 82$~keV. We derive the first experimentally-constrained thermonuclear reaction rate for ${}^{55}$Ni(p,${\hat{I}}^3$)${}^{56}$Cu. We find that, with this new rate, the rp-process may bypass the ${}^{56}$Ni waiting point via the ${}^{55}$Ni(p,${\hat{I}}^3$) 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.

8 pages, accepted for Phys. Rev. C