In Pursuit of Two-Proton Radioactivity

Ground-state two-proton (2p) radioactivity was discovered at the FRS facility at GSI Darmstadt 24 years ago. This decay mode can occur for isotopes of even-Z elements located beyond the two-proton drip line. So far, progress in experimental studies of this exotic process has been rather slow. In fact, it has been observed in only a few light- and medium-mass nuclides with Z ≤ 36. While theoretical models suggest that simultaneous two-proton emission should be observable for every heavier, even-Z element up to tellurium, most of the predicted candidates are extremely difficult to reach with present techniques.

One of the interesting questions in this context is to what extent the details of nuclear structure can be inferred from 2p decay observables. It is expected that momentum correlations between the emitted protons may shed light on the wave function of the decaying nucleus. Work is in progress to investigate whether the p-p correlations in the three classical cases — ⁴⁵Fe, ⁴⁸Ni, and ⁵⁴Zn — will reveal information about the Z = 28 shell closure in this region of the nuclear chart.

Recently, a significant breakthrough was achieved in this field. At the new Facility for Rare Isotope Beams (FRIB) at Michigan State University, our group succeeded in observing a large number of 2p decays for these three cases, increasing the available statistics for each by more than an order of magnitude. The data, collected with the help of the Warsaw Optical Time Projection Chamber (OTPC), are currently being analysed. In this talk, following a short overview of 2p radioactivity studies, I will present preliminary results from this remarkable experiment.