Shell evolution of $N=40$ isotones towards ${}^{60}$Ca: First spectroscopy of ${}^{62}$Ti

Abstract

Excited states in the $N=40$ isotone ${}^{62}$Ti were populated via the ${}^{63}$V$(p,2p)$${}^{62}$Ti reaction at $\sim$200~MeV/u at the Radioactive Isotope Beam Factory and studied using $\gamma$-ray spectroscopy. The energies of the $2_1^{+}\to 0_{\mathrm{gs}}^{+}$ and $4_1^{+}\to 2_1^{+}$ transitions, observed here for the first time, indicate a deformed ${}^{62}$Ti ground state. These energies are increased compared to the neighboring ${}^{64}$Cr and ${}^{66}$Fe isotones, suggesting a small decrease of quadrupole collectivity. The present measurement is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce our findings. The shell-model calculations for ${}^{62}$Ti show a dominant configuration with four neutrons excited across the $N=40$ gap. Likewise, they indicate that the $N=40$ island of inversion extends down to $Z=20$, disfavoring a possible doubly magic character of the elusive ${}^{60}$Ca.