High-Precision Mass Measurement of Cu 56 and the Redirection of the r p -Process Flow

We report the mass measurement of $^{56}\\mathrm{Cu}$, using the LEBIT 9.4 T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of $^{56}\\mathrm{Cu}$ is critical for constraining the reaction rates of the $^{55}\\mathrm{Ni}(p,\\ensuremath{\\gamma})$ $^{56}\\mathrm{Cu}(p,\\ensuremath{\\gamma})$ $^{57}\\mathrm{Zn}({\\ensuremath{\\beta}}^{+})$ $^{57}\\mathrm{Cu}$ bypass around the $^{56}\\mathrm{Ni}$ waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, $\\mathrm{ME}=\\ensuremath{-}38626.7(7.1)\\text{ }\\text{ }\\mathrm{keV}$, is a factor of 30 more precise than the extrapolated value suggested in the 2012 atomic mass evaluation [Chin. Phys. C 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 41, 030003 (2017)]. The new experimental average, using our new mass and the value from AME2016, is used to calculate the astrophysical $^{55}\\mathrm{Ni}(p,\\ensuremath{\\gamma})$ and $^{56}\\mathrm{Cu}(p,\\ensuremath{\\gamma})$ forward and reverse rates and perform reaction network calculations of the $rp$ process. These show that the $rp$-process flow redirects around the $^{56}\\mathrm{Ni}$ waiting point through the $^{55}\\mathrm{Ni}(p,\\ensuremath{\\gamma})$ 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.