Precision Mass Measurements of Neutron-Rich Scandium Isotopes Refine the Evolution of N=32 and N=34 Shell Closures

We report high-precision mass measurements of Sc50-55 isotopes performed at the LEBIT facility at NSCL and at the TITAN facility at TRIUMF. Our results provide a substantial reduction of their uncertainties and indicate significant deviations, up to 0.7 MeV, from the previously recommended mass values for 53-55SC. The results of this work provide an important update to the description of emerging closed-shell phenomena at neutron numbers N = 32 and N = 34 above proton-magic Z = 20. In particular, they finally enable a complete and precise characterization of the trends in ground state binding energies along the N = 32 isotone, confirming that the empirical neutron shell gap energies peak at the doubly magic Ca-52. Moreover, our data, combined with other recent measurements, do not support the existence of a closed neutron shell in Sc-55 at N = 34. The results were compared to predictions from both ab initio and phenomenological nuclear theories, which all had success describing N = 32 neutron shell gap energies but were highly disparate in the description of the N = 34 isotone.

Automated summary

High-precision mass measurements of Sc50-55 isotopes show significant deviations from previously recommended values, providing important updates for understanding closed-shell phenomena at neutron numbers N = 32 and N = 34 above proton-magic Z = 20. The results also indicate a lack of support for the existence of a closed neutron shell in Sc-55 at N = 34, and highlight discrepancies in predictions from different nuclear theories for the N = 34 isotone.