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

TL;DR

This study measures the beta-delayed proton emission of $^{57}$Zn, confirming it can bypass the $^{56}$Ni waiting point in the rp-process, which has implications for understanding nucleosynthesis in X-ray bursts.

Contribution

It provides the first definitive evidence that the $^{56}$Ni waiting point can be bypassed via the $^{57}$Zn decay pathway in the rp-process.

Findings

$^{57}$Zn half-life measured at 43.6 ms

Beta-delayed proton emission branching ratio is 84.7%

14-17% of the rp-process flow bypasses $^{56}$Ni

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 $f p$-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.