M1 and gamow-teller transitions in T = 1/2 nuclei ²³Na and ²³Mg

The magnetic dipole (M 1) operator consists not only of the usually dominant isovector (IV) spin (στ) term. but also of IV orbital (lτ), isoscalar (IS) spin (σ), and IS orbital (l) terms. On the other hand. the Gamow-Teller (GT) operator contains only the στ 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 M 1 transitions in the T = 1/2 mirror nuclei pair ²³Na-²³Mg, the contributions of these various terms have been studied by comparing the strengths of the analogous M1 γ transitions and the GT transitions deduced from high-resolution ²³Na(³He,t)²³Mg charge-exchange measurements. In some M 1 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 ²³Na and ²³Mg are deformed. The large difference of the orbital contributions is explained based on the different selection rules for l and σ operators in transitions connecting different deformed bands. Precise excitation energies are determined for the states above the proton separation energy in ²³Mg. These states are of astrophysical interest because of their important role played in the ²²Na(p, γ)²³Mg reaction of the Ne-Na cycle in nucleosynthesis.