${}^{57}$Zn ${\hat{I}}^2$-delayed proton emission establishes the ${}^{56}$Ni $rp$-process waiting point bypass

Abstract

We measured the ${}^{57}$Zn ${\hat{I}}^2$-delayed proton (${\hat{I}}^2$p) and ${\hat{I}}^3$ emission at the National Superconducting Cyclotron Laboratory. We find a ${}^{57}$Zn half-life of 43.6 $\pm$ 0.2 ms, ${\hat{I}}^2$p branching ratio of (84.7 $\pm$ 1.4)%, and identify four transitions corresponding to the exotic ${\hat{I}}^2$-${\hat{I}}^3$-$p$ decay mode, the second such identification in the $fp$-shell. The $p/{\hat{I}}^3$ 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.