Spin and Orbital Magnetic Quadrupole Resonances inC48aandZ90rfrom180°Electron Scattering

The nuclei ${}^{48}\mathrm{Ca}$ and ${}^{90}\mathrm{Zr}$ were investigated in $180°$ high-resolution inelastic electron scattering for momentum transfers $q\simeq 0.35–0.8$ ${\mathrm{fm}}^{-1\mathrm{}}$. The complete $M2\mathrm{}$ strength could be extracted in both nuclei up to excitation energies of about 15 MeV. Second-random-phase approximation calculations successfully describe the strong fragmentation of the experimental strength distributions. Contrary to previous experimental findings, suggesting a severe reduction, the deduced quenching of $M2\mathrm{}$ spin matrix elements is comparable to the $M1\mathrm{}$ case. A quantitative reproduction of the data requires the presence of appreciable orbital strength which can be interpreted as a torsional elastic vibration (“twist mode”).