β decay of v 61,63 and low-energy level schemes of Cr 61,63

Background: Near N = 40, rapid development of collectivity has been inferred based on the low-energy level schemes of the even-even Fe and Cr isotopes and attributed to deformation arising primarily from the influence of the $\ensuremath{\nu}{g}_{9/2}$ intruder orbital. The level schemes of the odd-A Co and Fe isotopes, as well as the odd-odd Co and Mn isotopes, are also influenced by the ${g}_{9/2}$ orbital and suggest prolate deformation. However, scarce information is available regarding the neutron-rich odd-A Cr isotopes.Purpose: Determine low-energy level schemes of the neutron-rich Cr isotopes approaching N = 40 and investigate the influence of the $\ensuremath{\nu}{g}_{9/2}$ orbital.Method: Neutron-rich V isotopes were produced at the NSCL through projectile fragmentation. The $\ensuremath{\beta}$ decay of the V isotopes into Cr isotopes was studied, and the observed $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays were used to determine the low-energy level schemes of the neutron-rich Cr isotopes.Results: A greatly expanded level scheme is constructed for $^{61}\mathrm{Cr}$, which has an increased low-energy level density relative to isotopic ${}^{55,57,59}$Cr. Excited states are discovered in $^{63}\mathrm{Cr}$ for the first time.Conclusion: The distinct difference between the low-energy level scheme of $^{61}\mathrm{Cr}$ and the lighter neutron-rich odd-A Cr isotopes is inferred to be due to the influence of the $\ensuremath{\nu}{g}_{9/2}$ intruder orbital and suggests the possibility of low-energy positive-parity states in $^{61}\mathrm{Cr}$, leading to the conclusion that a significant change in deformation and orbital occupancies has taken place when $N$ exceeds 36.