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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,

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Simultaneous investigation of the T = 1 (J π = 0 + ) and T = 0 (J π = 9 + ) β decays in The β decay of the odd-odd nucleus 70 Br has been investigated with the BigRIPS and EURICA setups at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center. The T = 0 (J π = 9 + ) and T = 1 (J π = 0 + ) isomers have both been produced in in-flight fragmentation of 78 Kr with ratios of 41.6(8)% and 58.4(8)%, respectively. A half-life of t 1/2 = 2157 +53 −49 ms has been measured for the J π = 9 + isomer from γ-ray time decay analysis. Based on this result, we provide a new value of the half-life for the J π = 0 + ground state of 70 Br, t 1/2 = 78.42 ± 0.51 ms, which is slightly more precise, and in excellent agreement, with the best measurement reported hitherto in the literature. For this decay, we provide the first estimate of the total branching fraction decaying through the 2 + 1 state in the daughter nucleus 70 Se, R(2 + 1 ) = 1.3 ± 1.1%. We also report four new low-intensity γ-ray transitions at 661, 1103, 1561, and 1749 keV following the β decay of the J π = 9 + isomer. Based on their coincidence relationships, we tentatively propose two new excited states at 3945 and 4752 keV in 70 Se with most probable spins and parities of J π = (6 + ) and (8 + ), respectively. The observed structure is interpreted with the help of shell-model calculations, which predict a complex interplay between oblate and prolate configurations at low excitation energies.

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