Polarization of 12B produced in 14N-induced reactions.

Abstract

Nuclear spin polarization of projectile-like fragments $^{12}\mathrm{B}$ produced in ${(}^{14}$N${,}^{12}$B) reactions at around the classical grazing angle was measured as a function of reaction Q value at incident energies of 120 and 200 MeV. Large positive $^{12}\mathrm{B}$ polarization was observed in the region of small kinetic energy loss for heavy target nuclei ranging from $^{232}\mathrm{Th}$ to $^{\mathrm{nat}}\mathrm{Fe}$, and it was interpreted in terms of direct two-proton transfer. For the target nucleus $^{27}\mathrm{Al}$, however, large negative polarization was observed in the region of small kinetic energy loss. It was found that the $^{45}\mathrm{Sc}$ target nucleus was situated at a crossover point for the sign of the polarization. This fact is explained by assuming coexistence of direct and frictional processes. It is found that the contribution of the frictional process tends to exceed that of the direct process with a decrease of target mass number in the region of small energy loss. In the region of large kinetic energy loss, the dominant mechanism is due to the frictional process. A friction constant of (2.4\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ MeV?${\mathrm{fm}}^{2}$ is deduced from the present experimental data. This value agrees in order of magnitude with that obtained from the recent study on the reaction Xe + Bi.