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

We report the mass measurement of Cu-56, using the LEBIT 9.4 T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of Cu-56 is critical for constraining the reaction rates of the Ni-55(p,gamma) Cu-56(p,gamma) Zn-57(beta(+)) Cu-57 bypass around the Ni-56 waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, ME = -38626.7(7.1) 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 Ni-55(p,gamma) and Cu-56(p,gamma) forward and reverse rates and perform reaction network calculations of the rp process. These show that the rp-process flow redirects around the Ni-56 waiting point through the Ni-55(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.