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

Abstract

Abstract 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.