Discovery of ^{72}Rb: A Nuclear Sandbank Beyond the Proton Drip Line.

In this Letter, the observation of two previously unknown isotopes is presented for the first time: Rb-72 with 14 observed events and Zr-77 with one observed event. From the nonobservation of the less proton-rich nucleus Rb-73, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For Rb-72, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, Rb-72, with a less exotic odd-even neighbor, Rb-73, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The Rb-72 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(+)(Rb-72) -> 9/2(+)(Kr-71) isomeric states with a broken mirror symmetry. These results imply that Kr-72 is a strong waiting point in x-ray burst rp-process scenarios.