A new study of the $N=32$ and $N=34$ shell gap for Ti and V by the first high-precision MRTOF mass measurements at BigRIPS-SLOWRI

Abstract

The atomic masses of 55 Sc, 56,58 Ti, and 56−59 V have been determined using the high-precision multi-reflection time-of-flight technique. The radioisotopes have been produced at RIKEN's RIBF facility and delivered to the newly combined novel designed gas cell and multi-reflection system (ZD-MRTOF), which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For 56,58 Ti and 56−59 V the mass uncertainties have been reduced down to the order of 10 keV, shedding new light on the N = 34 shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species 58 Ti and 59 V. With the new precision achieved, we reveal the non-existence of the N = 34 empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied νp 3/2 orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the νd 5/2 and νg 9/2 orbits and compare the results with conventional shell model calculations, which exclude the νg 9/2 and the νd 5/2 orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at N = 34.