Giant magnetic quadrupole resonance studied with 180 deg. electron scattering

The nuclei 48Ca and 90Zr were investigated in 180 deg. high-resolution inelastic electron scattering for momentum transfers q≅0.35-0.8 fm-1. Complete M2 strength distributions could be extracted in both nuclei up to excitation energies of about 15 MeV utilizing a fluctuation analysis technique. Second-RPA calculations successfully describe the experimentally observed strong fragmentation of the M2 mode. The quenching of the spin part is found to be comparable to the M1 case, contrary to previous claims suggesting a stronger reduction. A quantitative reproduction of the data requires the presence of appreciable orbital strength which can be interpreted as a torsional elastic vibration (the so-called twist mode)