Triplet of nuclear scissors modes

The low energy M1 excitations are studied within the Time Dependent Hartree-Fock-Bogoliubov (TDHFB) approach. The solution of TDHFB equations by the Wigner Function Moments method predicts three types of scissors modes. Together with the conventional scissors mode generated by the counter-rotation of protons against neutrons, two new modes arise due to spin degrees of freedom ("spin" scissors). Two states fall into the energy range of 2:7 < E < 3:7 MeV, adopted for the scissors mode. The lowest one generates a remarkable M1 strength below the conventional energy range. The results of calculations for mean excitation energies and summed excitation strengths of the scissors resonance in rare earth nuclei are presented. The main focus is on the low-lying magnetic dipole strength distribution in ^160,162,164Dy isotopes. A comparison with the results of systematic calculations within the Quasiparticle-Phonon Nuclear Model (QPNM) and with the data for integrated scissors resonance strength reported from nuclear resonance fluorescence and data of photoneutron measurement from the Oslo-type experiments is discussed. (author)