Discovery of Rb 72 : A Nuclear Sandbank Beyond the Proton Drip Line

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 ${5}^{+}\ensuremath{\rightarrow}5/{2}^{\ensuremath{-}}$ 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 ${9}^{+}(^{72}\mathrm{Rb})\ensuremath{\rightarrow}9/{2}^{+}(^{71}\mathrm{Kr})$ 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.