Tensor operator contribution to magnetic-dipole nuclear moments

Abstract

Excited states in atomic nuclei decay into lower-energy nuclear states through γ emission. These decays provide key information about nuclear structure and dynamics. This work studies magnetic dipole (M1) observables in medium-mass nuclei within the nuclear shell model. Since shell-model calculations are performed in a truncated valence space, the standard free-nucleon M1 operator requires effective corrections, and we include a tensor term to account for these effects. We derive analytically the one-body matrix elements of this operator and implement them in a state-of-the-art nuclear shell-model code. We focus on the calculation of magnetic moments for ground and excited states of several nuclei. We find that the tensor contribution is generally small (below 15%), but is enhanced when the magnetic moment of a state is close to zero. The relative weight of proton and neutron contributions to the tensor term is quite sensitive to the nuclear state