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: Rb72 with 14 observed events and Zr77 with one observed event. From the nonobservation of the less proton-rich nucleus Rb73, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For Rb72, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, Rb72, with a less exotic odd-even neighbor, Rb73, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The Rb72 half-life is consistent with a 5+→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+(Rb72)→9/2+(Kr71) isomeric states with a broken mirror symmetry. These results imply that Kr72 is a strong waiting point in x-ray burst rp-process scenarios.