High-precision mass measurement of $^{56}$Cu and the redirection of the rp-process flow

TL;DR

This study precisely measured the mass of $^{56}$Cu, significantly improving accuracy, and demonstrated its impact on the rp-process flow in astrophysical nucleosynthesis, showing a bypass around the $^{56}$Ni waiting point.

Contribution

The paper provides the most precise mass measurement of $^{56}$Cu to date and shows how this measurement alters the understanding of the rp-process flow in astrophysics.

Findings

The new $^{56}$Cu mass value is 30 times more precise than previous estimates.

Reaction network calculations indicate the rp-process bypasses the $^{56}$Ni waiting point.

The flow proceeds more quickly to higher masses, affecting nucleosynthesis outcomes.

Abstract

We report the mass measurement of $^{56}$Cu, using the LEBIT 9.4T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of $^{56}$Cu is critical for constraining the reaction rates of the $^{55}$Ni(p,$\gamma$)$^{56}$Cu(p,$\gamma$)$^{57}$Zn($\beta^+$)$^{57}$Cu bypass around the $^{56}$Ni waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, ME=$-38 626.7(6.4)$ keV, is a factor of 30 more precise than the suggested value from the 2012 atomic mass evaluation [Chin. Phys. C {\bf{36}}, 1603 (2012)], and more than a factor of 12 more precise than values calculated using local mass extrapolations, while agreeing with the newest 2016 atomic mass evaluation value [Chin. Phys. C {\bf{41}}, 030003 (2017)]. The new experimental average was used to calculate the astrophysical $^{55}$Ni(p,$\gamma$) and $^{57}$Zn($\gamma$,p) reaction rates and perform reaction network calculations of the rp-process. These show that the rp-process flow redirects around the $^{56}$Ni waiting point through the $^{55}$Ni(p,$\gamma$) route, allowing it to proceed to higher masses more quickly and resulting in a reduction in ashes around this waiting point and an enhancement to higher-mass ashes.