M1 and Gamow-Teller transitions in T=1/2 nuclei 23Na and 23Mg -: art. no. 044313

The magnetic dipole (M1) operator consists not only of the usually dominant isovector (IV) spin (sigmatau) term, but also of IV orbital (ltau), isoscalar (IS) spin (sigma), and IS orbital (l) terms. On the other hand, the Gamow-Teller (GT) operator contains only the sigmatau term. Under the assumption that isospin is a good quantum number, isobaric analog structure is expected in a pair of T=1/2 mirror nuclei, and thus analogous transitions are found. For the M1 transitions in the T=1/2 mirror nuclei pair Na-23-Mg-23, the contributions of these various terms have been studied by comparing the strengths of the analogous M1 gamma transitions and the GT transitions deduced from high-resolution Na-23(He-3,t)Mg-23 charge-exchange measurements. In some M1 transitions, unusually large orbital contributions were observed even for strong transitions, while in some others, almost no orbital contribution was found. It is known that Na-23 and Mg-23 are deformed. The large difference of the orbital contributions is explained based on the different selection rules for l and sigma operators in transitions connecting different deformed bands. Precise excitation energies are determined for the states above the proton separation energy in Mg-23. These states are of astrophysical interest because of their important role played in the Na-22(p,gamma)Mg-23 reaction of the Ne-Na cycle in nucleosynthesis.