${}^{57}$Zn $\beta$-delayed proton emission establishes the ${}^{56}$Ni $rp$-process waiting point bypass

Abstract

We measured the ${}^{57}$Zn $\beta$-delayed proton ($\beta$p) and $\gamma$ emission at the National Superconducting Cyclotron Laboratory. We find a ${}^{57}$Zn half-life of 43.6 $\pm$ 0.2 ms, $\beta$p branching ratio of (84.7 $\pm$ 1.4)%, and identify four transitions corresponding to the exotic $\beta$-$\gamma$-$p$ decay mode, the second such identification in the $fp$-shell. The $p/\gamma$ ratio was used to correct for isospin mixing while determining the ${}^{57}$Zn mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point ${}^{56}$Ni for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the ${}^{56}$Ni bypass, with 14-17% of the $rp$-process flow taking this route.

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• Dependence of X-Ray Burst Models on Nuclear Reaction Rates
• The Effects of Variations in Nuclear Processes on Type I X-Ray Burst Nucleosynthesis
• Nucleosynthesis in Type I X-ray Bursts
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• High-precision mass measurement of ${}^{56}$Cu and the redirection of the rp-process flow
• $\beta$-particle energy-summing correction for $\beta$-delayed proton emission measurements