Study of the N=32 and N=34 Shell Gap for Ti and v by the First High-Precision Multireflection Time-of-Flight Mass Measurements at BigRIPS-SLOWRI

The atomic masses of Sc55, Ti56,58, and V56-59 have been determined using the high-precision multireflection time-of-flight technique. The radioisotopes have been produced at RIKEN's Radioactive Isotope Beam Factory (RIBF) and delivered to the novel designed gas cell and multireflection system, which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For Ti56,58 and V56-59, 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 Ti58 and V59. With the new precision achieved, we reveal the nonexistence of the N=34 empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied νp3/2 orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the νd5/2 and νg9/2 orbits and compare the results with conventional shell model calculations, which exclude the νg9/2 and the νd5/2 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.