Precise Mass Measurement of Aluminum-22 Suggests Unlikely Proton-Halo Structure

The mass of an atomic nucleus reveals an interesting feature: it is always lighter than the constituent protons and neutrons. Through the nuclear strong force, a portion of the mass is converted into energy used to bind the nucleus together. The strength of this binding is critically important for many areas of nuclear science as well as stockpile stewardship considerations. This nuclear binding energy can only be measured experimentally, though many of the most interesting isotopes near the limits of existence are hard to produce and study. Thanks to recent advances, such as the Facility for Rare Isotope Beams (FRIB), we have unparalleled access to many new and exotic isotopes. Aluminum-22 is one such case: as the first proton-bound aluminum isotope, it was suggested to exhibit a very unusual ‘halo’ structure. I will present a recent mass measurement of aluminum-22 performed at FRIB, and review the latest theory, experiment, and implications for its nuclear structure.