The mechanism is studied which causes an anomalous magnetic moment of ${}^9\mathrm{C}$, i.e., too large isoscalar spin for the $T_z=\pm 3∕2$ mirror pair of $A=9$, from a shell-model viewpoint. Based on the empirically determined shell energies varying from stable to unstable nuclei, the effective $N=8$ shell gap for $Z=3$ is evaluated to be rather narrow in a reasonable way. Under this condition, an additional shell quenching by the Thomas-Ehrman effect, arising for ${}^9\mathrm{C}$ only, allows its ground state to be substantially mixed with the intruder configurations. This mirror asymmetry in the mirror wave functions accounts for the obviously large spin expectation value.

• Received 15 January 2004

DOI:https://doi.org/10.1103/PhysRevC.70.011303