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 $\nu g_{9/2}$ 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 $\nu g_{9/2}$ orbital.

Method: Neutron-rich V isotopes were produced at the NSCL through projectile fragmentation. The $\beta$ decay of the V isotopes into Cr isotopes was studied, and the observed $\beta$-delayed $\gamma$ 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 $\nu g_{9/2}$ 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.

• Received 10 May 2013
• Revised 21 January 2014

DOI:https://doi.org/10.1103/PhysRevC.89.034317