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,

Context. In a previous paper we investigated the energy transfer of massive stars to the interstellar medium (ISM) as a function of time and the geometrical configuration of three massive stars via 3D-mesh-refining hydrodynamics simulations, following the complete evolution of the massive stars and their supernovae with the exception of non-thermal processes. Aims. To compare our results against observations we derive thermal X-ray properties of the ISM from our simulations and compare them to observations of superbubbles in general, to the well-studied nearby Orion-Eridanus superbubble and to the diffuse soft X-ray emission of nearby galaxies. Methods. We analysed our ISM simulation results with the help of spectra for plasma temperatures between 0.1 and 10 keV and computed the spectral evolution and the spatio-temporal distribution of the hot gas. Results. Despite significant input of high-temperature gas from supernovae and fast stellar winds, the resulting thermal X-ray spectra are generally very soft, with most of the emission well below 1 keV. We show that this is due to mixing triggered by resolved hydrodynamic instabilities. Supernovae enhance the X-ray luminosity of a superbubble by 1-2 orders of magnitude for a time span of about 0.1 Myr; which is longer if a supernova occurs in a larger superbubble and shorter in higher energy bands. Peak superbubble luminosities of the order of 10 36 erg s −1 are reproduced well. The strong decay of the X-ray luminosity is due to bubble expansion, hydrodynamic instabilities related to the acceleration of the superbubble's shell thanks to the sudden energy input, and subsequent mixing. We also find global oscillations of our simulated superbubbles, which produce spatial variations of the X-ray spectrum, similar to what we see in the Orion-Eridanus cavity. We calculated the fraction of energy emitted in X-rays and find that with a value of a few times 10 −4 , it is about a factor of ten below the measurements for nearby galaxies. Conclusions. Our models explain the observed soft spectra and peak X-ray luminosities of individual superbubbles. Each supernova event inside a superbubble produces a fairly similar heating-entrainment-cooling sequence, and the energy content of superbubbles is always determined by a specific fraction of the energy released by one supernova. For a given superbubble, soft X-rays trace the internal energy content well with moderate scatter. Some mechanism seems to delay the energy loss in real superbubbles compared to our simulations. Alternatively, some mechanism other than thermal emission of superbubbles may contribute to the soft X-ray luminosity of star-forming galaxies.

A record number of 100 Sn nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the β-decay end point energy of 100 Sn were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0 þ → 1 þ Gamow-Teller decay was improved to B GT ¼ 4.4 þ0.9 −0.7 . A discrimination between different model calculations was possible for the first time, and the level scheme of 100 In is investigated further.Sn and its neighboring nuclei comprise a unique testing ground for modern large scale shell model (LSSM) calculations with realistic nuclear interactions. 100 Sn is the heaviest doubly magic N ¼ Z nucleus that is particle stable and decays via a pure and very fast Gamow-Teller (GT) β decay. The 100 Sn region is located in the nuclear chart close to the end of the astrophysical rapid proton capture process path. Thus, it is of particular interest concerning fundamental challenges in both nuclear physics and astrophysics [1].According to the extreme single particle model (ESPM) [2], 100 Sn decays via a pure GT transition of a proton (π) from the completely filled π0g 9=2 orbital into a neutron (ν) in the empty spin-orbit partner, the ν0g 7=2 orbital of 100 In. The ESPM GT strength is predicted to be B GT ¼ 17.78 [1]. However, the experimental values obtained up to now are smaller: 9.1 þ3.0 −2.6 [3] and 5.8 þ5.5 −3.2 [4,5]. These experiments [3,5,6] revealed the smallest log(ft) value-even smaller than the values of nuclei which decay by a Superallowed Fermi decay-throughout the nuclear chart. However, the PHYSICAL REVIEW LETTERS 122, 222502 (2019) 0031-9007=19=122 (22)=222502(6) 222502-1

Half-lives and energies of γ rays emitted in the decay of isomeric states of nuclei in the vicinity of the doubly magic 100 Sn were measured in a decay spectroscopy experiment at Rikagaku Kenkyusho (The Institute of Physical and Chemical Research) of Japan Nishina Center. The measured half-lives, some with improved precision, are consistent with literature values. Three new results include a 55-keV E2 γ ray from a new (4 + ) isomer with T 1/2 = 0.23(6) μs in 92 Rh, a 44-keV E2 γ ray from the (15 + ) isomer in 96 Ag, and T 1/2 (6 + ) = 13(2) ns in 98 Cd. Shell-model calculations of electromagnetic transition strengths in the (p 1/2 ,g 9/2 ) model space agree with the experimental results. In addition, experimental isomeric ratios were compared to the theoretical predictions derived with an abrasion-ablation model and the sharp cutoff model. The results agreed within a factor of 2 for most isomers. From the nonobservation of time-delayed γ rays in 100 Sn, new constraints on the T 1/2 , γ -ray energy, and internal conversion coefficients are proposed for the hypothetical isomer in 100 Sn.

Several new isotopes, 96 In, 94 Cd, 92 Ag, and 90 Pd, have been identified at the RIKEN Nishina Center.The study of proton drip-line nuclei in the vicinity of 100 Sn led to the discovery of new proton emitters 93 Ag and 89 Rh with half-lives in the submicrosecond range. The systematics of the half-lives of odd-Z nuclei with T z ¼ −1=2 toward 99 Sn shows a stabilizing effect of the Z ¼ 50 shell closure. Production cross sections for nuclei in the vicinity of 100 Sn measured at different energies and target thicknesses were compared to the cross sections calculated by EPAX taking into account contributions of secondary reactions in the primary target.