In this Letter, the observation of two previously unknown isotopes is presented for the first time: ${}^{72}\mathrm{Rb}$ with 14 observed events and ${}^{77}\mathrm{Zr}$ with one observed event. From the nonobservation of the less proton-rich nucleus ${}^{73}\mathrm{Rb}$, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For ${}^{72}\mathrm{Rb}$, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, ${}^{72}\mathrm{Rb}$, with a less exotic odd-even neighbor, ${}^{73}\mathrm{Rb}$, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The ${}^{72}\mathrm{Rb}$ half-life is consistent with a $5^{+}\to 5/2^{-}$ proton decay with an energy of 800–900 keV, in agreement with the atomic mass evaluation proton-separation energy as well as results from the finite-range droplet model and shell model calculations using the GXPF1A interaction. However, we cannot explicitly exclude the possibility of a proton transition between $9^{+}({}^{72}\mathrm{Rb})\to 9/2^{+}({}^{71}\mathrm{Kr})$ isomeric states with a broken mirror symmetry. These results imply that ${}^{72}\mathrm{Kr}$ is a strong waiting point in x-ray burst $rp$-process scenarios.

• Received 3 April 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.192503