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

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,${\hat{I}}^3$)${}^{56}$Cu(p,${\hat{I}}^3$)${}^{57}$Zn($β^+$)${}^{57}$Cu bypass around the ${}^{56}$Ni waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, ME=$-38626.7(6.4)$ 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,${\hat{I}}^3$) and ${}^{57}$Zn(${\hat{I}}^3$,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,${\hat{I}}^3$) 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.

6 pages, 5 figures