Weakening of N=28 shell gap and the nature of 0₂ ⁺ states

The work reports a novel application of the interacting boson model in light-mass region around 48 Ca, that takes into account intruder states and configuration mixing. The model is shown to provide a reasonable description of the observed low-lying yrast and yrare states of the N = 28 even–even isotones from Si to Fe, and even–even Ca isotopes. The nature of 0 2 + states is addressed in terms of the competition between spherical and deformed intruder configurations, and the rigidity of the N = 28 shell gap is tested, particularly for 44 S. The 0 2 + isomer in 44 S is shown to arise from a weak mixing between the two configurations and called as shape isomer. The unresolved low-lying spectra in 46 Ar is also approached using the core-excitations across N = 28 shell. The SU(3) structure in 42 Si is supported by the present calculation. The nearly spherical nature of 50 Ti, 52 Cr, 54 Fe and 42,44,46 Ca isotopes is found, while the core excitations are found to be essential for the description of yrare states. Shell model calculation is also performed for comparison.