Near- and Far-Side Contributions to Ejectile Polarization

Abstract

Near· and far·side contributions to the ejectile polarization phenomena are studied by the use of Quantum Molecular Dynamics plus external mean field model. It is shown that the far-side contribution increases as the incident energy increases or the target charge decreases. The incident energy and the target dependence of ejectile polarization data is reproduced qualitatively. The near­ and far-side contributions themselves are calculated to be almost monotonic functions of ejectile momentum as is predicted in a simple projectile fragmentation picture, and their statistical average results in various shapes in ejectile ,polarization. The projectile fragmentation is now an important way to get polarized unstable nuclear beams. Asahi et aL have shown that this method is useful, and that in the reaction of 14N + 197 Au --> 12B + X(40 MeV lu), the ejectile spin polarization is well understood by a simple picture on the assumption that the near-side contribution is dominant I) Asahi's picture relates the momentum of the removed nucleons and the ejectile spin. In his picture, when the removed nucleons have a total momentum K and their center is R in the center-of-mass system of the projectile, the ejectile will have a momentum PF = - K and an angular momentum J F = - R X K on the assumption that the linear and angular momentum transfers between the projectile and the target are negligible. Then the ejectile polarization becomes a monotonic function of ejectile momentum and its slope is positive if the near-side process is dominant. I) In the above reaction, the ejectile polarization is observed to be a monotonically increasing function of ejectile momentum and the polarization data is well reproduced by a simple estimate based on Asahi's picture and Glauber's multiple scattering theory.IH) This suggests to us that the nuclear orbital which mainly contributes to this reaction is near-side one, and that the linear and angular momentum transfers between the projectile and the target can be actually ignored. On the other hand, Asahi's picture shows us that the polarization slope becomes negative in the far-side dominance case, and this was experimentally observed in the nitrogen induced reactions on 93Nb and 26,27 Al targets. 3 ) Due to the classical nature of this picture, it is expected that the picture is also valid for higher incident energies. In the gold target reaction at 69.5 MeV lu, how­ ever, the ejectile polarization is not a monotonic function of ejectile momentum. 3 ) Namely, there appears a dip around beam velocity. One of the probable origins of this behavior is the mixing of the near-side and far-side contributions to the ejectile polarization. However, it has not been understood yet when and where the near- or far-side contribution becomes dominant and the mixing of them becomes important to