Rotational model and shell model pictures of magnetic dipole excitations

A comparison is made of the rotational model and the large shell model for the calculation of the magnetic dipole excitations in selected nuclei in the s-d and f-p shells. Focus is given on both the relatively low lying strong states and higher ''spin flip'' excitations. It is found that the two models agree fairly well for summed B(M1) strength, but for the deformations that were used give different results for orbital and spin decomposition. The rotational model yields too much low lying strength as compared with the shell model. A technique for eliminating spurious contributions to the summed strength in the rotational model is developed. It is shown that in the asymptotic limit in some nuclei the magnetic dipole excitations are purely orbital. The implications of these calculations to results in heavier deformed nuclei for ''scissor mode'' excitations is discussed