Effects of the spin-orbit and tensor interactions on M1 excitations in light nuclei

The effects of varying the spin-orbit and tensor components of a realistic interaction on M1 excitation rates are studied in nuclei in the 0 p and 1 s-0 d shells. Besides the total B( M1) strength also the spin and orbital parts are studied separately. The calculations are self-consistent in that the single-particle energies are obtained with the same interaction that is used between the valence nucleons. For most nuclei (with the possible exception of the upper half of the 1 s-0 d shell) a much better agreement with the experimental B( M1) rates is achieved by increasing the strength of the spin-orbit interaction relative to the free value. Weakening the tensor interaction relative to the free value in general also improves the comparison with the data, but the effects are less pronounced. Raising the 1 s relative to the 0 d single-particle level leads to an improved description of energies and B( M1) transition strengths of 1 ⁺ states. On a more subtle level, the strength of the spin-orbit interaction in the lower half of the sd shell as determined by the optimum description of M1 excitations leads to a splitting substantially larger than the experimentally observed difference E(J ^π = {3}/{2 ₁⁺}) - E(J ^π = {5}/{2 ₁⁺}) in ¹⁷O but this calculated splitting is in line with the (2 l + 1) systematics. Also studied is how much of the M1 orbital and spin strengths lie in an observable region and how much are buried in the grass at higher energies. It is noted that for many nuclei the sum of orbital B( M1) plus the sum of the spin B( M1) strengths is very close to the sum of the total B( M1) strength; thus the summed cross terms are small, although they are not so individually.