Results from the precision measurement of the mass of the possible proton halo candidate 22 Al and outlook on the physics of neutron deficient nuclei explored by FRIB.

Neutron-deficient nuclei form a rich laboratory for nuclear structure and nuclear astrophysics. For example, along the N = Z line, the protons and neutrons in these nuclei occupy the same shell model orbitals, resulting in a large spatial overlap of single-particle wavefunctions and, therefore, permit unique isospin studies. In addition, midway along the N = Z line between the double-shell closures at 56 Ni and 100 Sn, a rapid change in nuclear shape is observed, giving rise to the most deformed ground states in the nuclear chart. The astrophysical rp-process, which runs along the N = Z line, fuels Type I X-ray bursts. The accurate interpretation of observed X-ray burst light curves requires both precision measurements of ground and excited state properties. Finally, the emergence of exotic nuclear structure phenomena, such as proton halos where the wavefunctions of one or more protons extend far beyond a compact nuclear core, provide stringent test cases for nuclear models. A unifying feature of all these topics is their sensitivity to the nuclear binding energy that can be studied through mass spectroscopy.

The Low-Energy Beam and Ion Trap (LEBIT) facility [1] at the recently commissioned Facility for Rare Isotope Beams (FRIB) remains the only facility that employs Penning trap mass spectrometry for high-precision mass measurements of rare isotopes produced via projectile fragmentation, a technique that excels at producing neutron- deficient rare isotopes. This powerful combination of a fast, chemically insensitive rare isotope production method with a high-precision Penning trap mass spectrometer has yielded mass measurements of short-lived rare isotopes with precisions below 10 ppb across the chart of nuclides. The first LEBIT mass measurement campaign in the FRIB era was a mass measurement of the proton dripline nucleus 22 Al [2], a potential proton halo candidate. In this talk I will discuss the results of this experiment, as well as prospects for future exploration in the neutron-deficient sector of the nuclear chart at FRIB.

[1] R. J. Ringle, S. Schwarz, and G. Bollen, Int. J. Mass Spectrom. 349-350, 87 (2013).

[2] S. Campbell, et al., Phys. Rev. Lett. 132, 152501 (2024).