g factor of a magnetic-rotational band head in Pb-193

The recently discovered regular bands of enhanced M1 gamma-ray transitions in nearly spherical Pb nuclei have been explained in terms of magnetic rotation. However. the special angular momentum coupling of the nucleons that form magnetic-rotational states has never been tested experimentally. In this paper, we report on the measurement of the g factor of the 2583-keV head of such a band in Pb-193. The result, g = 0.68(3), is the first direct proof of the almost perpendicular coupling of the proton-particle and neutron-hole angular momentum vectors. This coupling results in a large component of the magnetic moment perpendicular to the total nuclear angular momentum, which is required for the occurrence of magnetic rotation.