$^{61}$Cr as a Doorway to the N = 40 Island of Inversion
L. Lalanne, M. Athanasakis-Kaklamanakis, D. D. Dao, Á. Koszorús, Y. C. Liu, R. Mancheva, F. Nowacki, J. Reilly, C. Bernerd, K. Chrysalidis, T. E. Cocolios, R. P. de Groote, K. T. Flanagan, R. F. Garcia Ruiz, D. Hanstorp, R. Heinke, M. Heines, P. Lassegues, K. Mack, B. A. Marsh, A. McGlone, K. M. Lynch, G. Neyens, B. van den Borne, R. Van Duyse, X. F. Yang, J. Wessolek
Published: 2024/9/11
Abstract
This paper reports on the measurement of the ground-state spin and nuclear magnetic dipole moment of $^{61}$Cr. The radioactive ion beam was produced at the CERN-ISOLDE facility and was probed using high-resolution resonance ionization laser spectroscopy with the CRIS apparatus. The present ground-state spin measurement $I = \frac{1}{2}$, differing from the previously adopted $I =(\frac{5}{2})$, has significant consequences on the interpretation of existing beta decay data and nuclear structure in the region. The structure and shape of $^{61}$Cr is interpreted with state-of-the-art Large-Scale Shell Model and Discrete-Non-Orthogonal Shell Model calculations. From the measured magnetic dipole moment $\mu(^{61}$Cr$)=+0.539(7)~\mu_N$ and the theoretical findings, its configuration is understood to be driven by 2 particle - 2 hole neutron excitations with an unpaired $1p_{1/2}$ neutron. This establishes the western border of the $N=40$ Island Of Inversion (IoI), characterized by 4 particle - 4 hole neutron components. We discuss the shape evolution along the Cr isotopic chain as a quantum phase transition at the entrance of the $N=40$ IoI.