SCISSORS MODES AND SPIN EXCITATIONS IN LIGHT-NUCLEI INCLUDING DELTA-N=2 EXCITATIONS - BEHAVIOR OF BE-8 AND BE-10

Shell model calculations are performed for magnetic dipole excitations in Be-8 and Be-10, first with a quadrupole-quadrupole interaction (Q . Q) and then with a realistic interaction. The calculations are perfo rmed both in a 0p space and in a large space which includes all <2(h)o ver bar omega> excitations. In the 0p with Q . Q we have an analytic e xpression for the energies of all states. In this Limit we find that i n Be-10 the L = 1 S = 0 scissors mode with isospin T = 1 is degenerate with that of T = 2. By projection from an intrinsic state we can obta in simple expressions for B(M1) to the scissors modes in Be-8 and Be-1 0. We plot cumulative sums for energy-weighted isovector orbital trans itions from J = 0(+) ground states to the 1(+) excited states. These h ave the structure of a low-energy plateau and a steep rise to a high-e nergy plateau. The relative magnitudes of these plateaux are discussed . By comparing Be-8 and Be-10 we find that contrary to the behaviour i n heavy deformed nuclei B(M1)(orbital) is not proportional to B(E2). O n the other hand, a sum rule which relates B(M1) to the difference (B( E2)(isoscalar) - B(E2)(isovector)) succeeds in describing the differen ce in behaviours in the two nuclei. The results for Q . Q and the real istic interactions are compared, as are the results in the 0p space an d the large (0p + <2(h)over bar omega>) space. The Wigner supermultipl et scheme is a very useful guide in analyzing the shell model results. (C) 1996 Academic Press, Inc.