Magnetic dipole transitions in 32S from electron scattering at 180° -: art. no. 024311

Magnetic dipole transitions in the self-conjugate nucleus S-32 up to an excitation energy of 12 MeV have been investigated in inelastic electron scattering at Theta = 180degrees at the superconducting Darmstadt electron linear accelerator (S-DALINAC). Transition strengths have been determined from a plane-wave Born approximation analysis including Coulomb distortion. For the two strongest M1 transitions, where a discrepancy of a factor of about 2 was observed in previous (e,e') experiments, values intermediate between the two extremes are deduced from the present work. The resulting strength distribution is well described by shell-model calculations using the unified sd-shell interaction and an effective M1 operator. The shell-model wave functions also provide a reasonable description of the form factors. A quasiparticle random phase approximation calculation is less successful. The present results allow for the first time studies of the form factor of extremely weak l-forbidden and isoscalar M1 excitations in S-32. The l-forbidden transition allows a sensitive test of tensor corrections to the M1 operator. A combined analysis with the isospin-analog Gamow-Teller (GT) transitions in the A = 32 triplet reveals a situation similar to previous studies in A = 39 nuclei: microscopic calculations reasonably account for the GT strengths, but fail in the case of M1 strengths. A possible explanation may be found in the nonrelativistic treatment of the latter. Some examples of the role of relativistic corrections are discussed. A consistent description of the reduced transition strength and the form factor of the isoscalar M1 excitation requires isospin mixing with the close-lying isovector transitions. The extracted Coulomb matrix elements are roughly within the limits set by the approximate constancy of the spreading width derived from the analysis of compound-nucleus reactions.