In an experiment with the BigRIPS separator at the RIKEN Nishina Center, we observed two-proton (2p) emission from 67 Kr. At the same time, no evidence for 2p emission of 59 Ge and 63 Se, two other potential candidates for this exotic radioactivity, could be observed. This observation is in line with Q value predictions which pointed to 67 Kr as being the best new candidate among the three for two-proton radioactivity. 67 Kr is only the fourth 2p ground-state emitter to be observed with a half-life of the order of a few milliseconds. The decay energy was determined to be 1690(17) keV, the 2p emission branching ratio is 37(14)%, and the half-life of 67 Kr is 7.4(30) ms. DOI: 10.1103/PhysRevLett.117.162501 Close to the valley of β stability, nuclear β decay, which is often associated with γ-ray emission, is the only decay mode possible. When moving closer to the limits of stability in both directions, the available decay energy, the Q value, increases at the same time as the binding energy of the excess particle decreases. Therefore, emission of β-delayed particles (protons, neutrons, or α particles) becomes more and more likely. Close to the proton drip line, β-delayed one-, two-, and (in particular recently) three-proton emission has been observed [1][2][3][4][5][6].In all these cases, the excess protons are still sufficiently bound that direct particle emission is not possible.However, when moving further away from the line of stability, the protons are no longer bound by the strong nuclear force and the proton drip line is crossed. For slightly negative proton separation energies S p or S 2p , β þ decay can still compete with direct one-or two-proton emission; however, with separation energies typically below −1 MeV, one-and two-proton emission dominates for odd-and even-Z elements, respectively. We underline here that for 2p radioactivity, the one-proton separation energy has to be positive.For odd-proton-number (odd-Z) elements, one-proton radioactivity is a well-established decay mode and is PRL 117,
