The puzzling two-proton decay of 67Kr

Two-proton (2p) radioactivity is a rare decay mode found in a few nuclei beyond the proton dripline. In such nuclei, the emission of a single proton is energetically forbidden or strongly suppressed due to the odd-even binding energy effect originating from proton pairing. The 2p-decay lifetime and properties of emitted protons carry invaluable information on the structure of an unbound nuclide. The 2p decay of ⁶⁷Kr, recently measured,  turned out to be unexpectedly fast. The recent paper by Research Associate Simin Wang et al. published in Phys. Rev. Lett.  offers an explanation to the puzzle.

The rare isotope ⁶⁷Kr is expected to be a deformed system that can undergo collective rotation, see Fig. 1. In the first application of a three-body approach to a two-nucleon decay from a deformed nucleus, the authors employed the recently developed theoretical framework. They demonstrated that shape deformation significantly reduces the 2p lifetime of ⁶⁷Kr, in agreement with experimental data.

The predicted sensitivity of the 2p lifetime to the proton-proton interaction indicates that the pairing between the valence protons can strongly influence the decay process. By studying proton decay energies and angular correlations, the paper concludes that there the observed decay is impacted by a competition between 2p and 1p decay modes.

The territory of 2p radioactivity is vast: it is expected that almost all elements between argon and lead have 2p-decaying isotopes [Olsen et al., 2013]. The future theoretical work will primarily focus on improving the quality of the interaction governing the decay process. To this end, high-statistics angular correlation 2p data are needed to better constrain theoretical input and improve our understanding of 2p radioactivity.

Fig. 1 A schematic diagram illustrating two protons emitted by an oblate-deformed nucleus, which can rotate around the axis perpendicular to its symmetry axis.