1 Non-Scissors-Mode Behaviour of Isovector Magnetic Dipole Orbital Transitions Involving Isospin Transfer

We study the response of isovector orbital magnetic dipole (IOMD) tran-sitions to the quadrupole-quadrupole (Q · Q) interaction, to the isospin-conserving pairing interaction (ICP) and to combinations of both. We find qualitatively different behaviours for transitions in which the final isospin dif-fers from the initial isospin versus cases where the two isospins are the same. For N = Z even-even nuclei with Jpi = 0+, T = 0 ground states such as 8Be and 20Ne, the summed T = 0 → T = 1 IOMD from the ground state to all the J = 1, T = 1 states in the 0h̄ω space does not vanish when the Q · Q interaction is turned off. The pairing interaction (ICP) alone leads to a finite transition rate. For nuclei with J = 0+, T = 1 ground states such 2 as 10Be and 22Ne, the summed T = 1 → T = 1 IOMD does vanish when the Q · Q interaction is turned off, as is expected in a good scissors-mode behaviour. However this is not the case for the corresponding sum of the T = 1 → T = 2 IOMD transitions. In 22Ne (but not in 10Be) the sum of the T = 1 → T = 2 IOMD transitions is remarkably insensitive to the strengths of both the Q ·Q and the ICP interactions. In 22Ne an energy weighted-sum is similarly insensitive. All our calculations were carried out in the 0h̄ω space. I