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Dawning of nuclear magicity in N = 32 seen through precision mass spectrometry Leistenschneider, Erich


In the early days of nuclear science, physicists were astounded that specific "magic" combinations of neutrons or protons within nuclei seemed to bind together more tightly than other combinations. This phenomenon was related to the formation of shell structures in nuclei. More recently, nuclear shells were observed to emerge or vanish as we inspect nuclei further from stability. The structural evolution of these changing shells has been the object of intense experimental investigation, and their behavior has become a standard ruler to benchmark theoretical predictions. In this work, we investigated the emergence of shell effects in systems with 32 neutrons (N = 32) using mass spectrometry techniques. Evidence for "magicity" was observed in potassium (with 19 protons, or Z = 19), calcium (Z = 20) and scandium (Z = 21), but not in vanadium (Z = 23) and higher-Z elements. In between, the picture at titanium (Z = 22) was unclear. We produced neutron-rich isotopes of titanium and vanadium through nuclear reactions at the ISAC facility and measured their atomic masses at the TITAN facility, in the TRIUMF Laboratory in Vancouver. These measurements were performed with the newly commissioned Multiple-Reflection Time-of-Flight Mass Spectrometer at TITAN facility and were substantiated by independent measurements from the Penning trap mass spectrometer. The atomic masses of ⁵²⁻⁵⁵Ti and ⁵²⁻⁵⁵V isotopes were measured with high precision, right at the expected emergence of N = 32 shell effects. Our results conclusively establish the existence of weak shell effects in titanium and confirm their absence in vanadium. Calculations of the N = 32 nuclear shell are within reach of the so-called ab initio theories. In these, complex atomic nuclei are described theoretically from fundamental principles, by applying principles of Quantum Chromodynamics to many-body quantum methods. Our data were compared with a few state-of-the-art ab initio calculations which, despite very successfully describing the N = 32 shell effects in Ca and Sc isotopes, overpredict its strength in Ti and erroneously assign V as its point of appearance. We hope the deficiencies revealed by our work will guide the development of the next generation of ab initio theories.

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