Low-lying level structure of ~(56)Cu and its implications for the rp process

摘要

The low-lying energy levels of proton-rich ~(56)Cu have been extracted using in-beam γ -ray spectroscopy with the state-of-the-art γ -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,γ )~(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 isobaric multiplet mass equation (IMME) fit is used, resulting in Q = 639 ± 82 keV. We derive the first experimentally constrained thermonuclear reaction rate for ~(55)Ni(p,γ )~(56)Cu.We find that,with this newrate, the rp processmay bypass the ~(56)Niwaiting point via the ~(55)Ni(p,γ ) reaction for typical x-ray burst conditions with a branching of up to ～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.