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 55Sc, 56'58Ti, and 56-59V 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 56'58Ti and 56-59V, 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 58Ti and 59V. 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 vp3/2 orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the vd5/2 and vg9/2 orbits and compare the results with conventional shell model calculations, which exclude the vg9/2 and the vd5/2 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.