Zakopane 2026 Conference on Nuclear Physics

The limit of existence on the neutron-deficient side of the nuclide chart is determined by the proton separation energy. In his 1960 work [1], V. Goldansky postulated the probability of simultaneous emission of two protons, known today as two-proton (2p) radioactivity, for even-Z isotopes beyond the proton drip line. It is an extremely rare phenomenon and to this day only four 2p emitters were observed: $^{45}$Fe [2 - 5], $^{48}$Ni [6, 7], $^{54}$Zn [8 - 10] and $^{67}$Kr [11]. However, with the exception of $^{45}$Fe, the global statistics till now is by far insufficient to even attempt at understanding this decay mode, preventing to draw any conclusions about decay mechanism or insight into competition between $\beta^+$ and 2p decay.

At the beginning of 2025, a series of measurements of the decays of $^{48}$Ni, $^{54}$Zn, and $^{45}$Fe were conducted at the Facility for Rare Isotope Beams (FRIB) [12] at Michigan State University with the Warsaw OTPC (Optical Time Projection Chamber) detector. The ions of interest were produced in fragmentation of a primary $^{58}$Ni beam on a carbon target, separated by the Advanced Rare Isotope Separator (ARIS) spectrometer [13], transmitted down the beamline and implanted into the OTPC detector, where subsequent decays were registered.

These measurements allow for the study of the structure of these even-Z nuclei beyond the proton drip-line, in particular the p-p correlation pattern across the Z=28 shell closure. While the analysis of the data is in progress, preliminary results can already provide valuable insight into decay properties and will be presented.

[1] V. Goldansky, Nuclear Physics 19 (1960): 482-495.
[2] M. Pfützner et al., EPJ A 14.3 (2002): 279-285.
[3] J. Giovinazzo et al., PRL 89.10 (2002): 102501.
[4] C. Dossat et al., PRC (2005): 054315.
[5] K. Miernik et al., PRL 99.19 (2007): 192501.
[6] M. Pomorski et al., PRC 90.1 (2014): 014311.
[7] A. Ortega Moral et al., PRC 112.6 (2025): L061302.
[8] B. Blank et al., PRL 94.23 (2005): 232501.
[9] P. Ascher et al., PRL 107.10 (2011): 102502.
[10] A. Kubiela et al., PRC 113.2 (2026): 024310.
[11] T. Goigoux et al., PRL 117.16 (2016): 162501.
[12] T. Glasmacher et al., Nuclear Physics News 27.2 (2017): 28-33
[13] M. Hausmann et al, NIM B 317 (2013): 349-353.