Two-Proton Radioactivity ofKr67

Abstract

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,

Puzzling Two-Proton Decay of Kr67

“…At the deformation β 2 ≈ −0.3, which is consistent with estimates from mirror nuclei [62] and various calculations [45][46][47][48]62] the calculated 2p g.s. half-live of 67 Kr is 24 ms, which agrees with experiment [1]. Since the Minnesota force used here is an effective in-teraction that is likely to be affected by in-medium effects, one may ask how changes in the proton-proton interaction may affect the 2p decay process.…”

“…As suggested in Ref. [1], this may be due to configuration mixing effects and/or deformation in the daughter nucleus 65 Se. An alternative explanation involves the competition between two-body and threebody decay channels [25]: the partial 2p lifetime can be reproduced only if the two valence protons primarily occupy the 2p 3/2 shell that is supposed to be already filled by the core nucleons.…”

“…The 2pdecay lifetime and properties of emitted protons carry invaluable information on nuclear structure in the presence of low-lying proton continuum. The recently measured 2p decay of 67 Kr [1] turned out to be unexpectedly fast. Since 67 Kr is expected to be a deformed system, we investigate the impact of deformation effects on the 2p radioactivity.…”

Recent developments in radioactive charged-particle emissions and related phenomena

“…In Ref. [72], the 2p decay from 67 Kr was observed. The decay energy was determined to be 1690(17) keV, the 2p decay branching ratio is 37(14)%, and the half-life is 7.4(30) ms which is considerably below earlier theoretical expectations.…”

“…[64]. On the experimental side, the 2p decay was firstly observed in 45 Fe [65,66] and then in 19 Mg [67], 48 Ni [68,69] , 54 Zn [70,71], and 67 Kr [72]. The possible observation of two-proton decay from intermediate-mass nuclei was discussed recently in Ref.…”