Weakening of $N=28$ shell gap and the nature of $0_2^{+}$ states

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. The $0_2^{+}$ isomer in ${}^{44}$S is shown to arise from a weak mixing between the two configurations. 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.