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

We study the response of isovector orbital magnetic dipole (IOMD) transitions to the quadrupole-quadrupole (Q · Q) interaction, to the isospinconserving pairing interaction (ICP) and to combinations of both. We find qualitatively different behaviours for transitions in which the final isospin differs from the initial isospin versus cases where the two isospins are the same. For N = Z even-even nuclei with J = 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.