TWO-PROTON RADIOACTIVITY OF ⁶⁷Kr

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,

Extended R-matrix description of two-proton radioactivity

“…The 2p decay in 19 Mg was observed with different techniques at GSI due to its much shorter half-life [7]. Finally, the very recent case of 67 Kr was reported in an experiment at RIKEN in Japan with a silicon detector telescope [8].…”

“…The emitted two protons have a small angular distribution and share the same energy. Until now, only five nuclei have been observed that can have 2p radioactivity from the ground state ( 45 Fe [2,4], 54 Zn [5], 48 Ni [6], 19 Mg [7], and 67 Kr [8]). A systematic study of 2p radioactivity with density functional theory suggests that this decay mode may not be limited to the light-and mediummass regions but a typical feature for even-Z proton-unbound isotopes, and should be observed for nuclei in the heavier-mass region [9].…”

Possible 2p Decay Emission in the Region 4 ≤ Z ≤ 54 Using the Modified CYE Model

“…Using the invariant mass approach, the 2p decays of the 12 O [19] and 16 Ne [20] were experimentally studied. Eventually, in 2016, the French group at the fragment separator Big RIPS in RIKEN [21] observed the fourth 2p emitter, 67 Kr with a half-lifetime of a few milliseconds. Another 2p-unbound isotope 11 O has been discovered in 2019 [22].…”