Alpha-transfer reactions in light nuclei. II. (He3,Be7) pickup reactions

Abstract

The ($^{3}\mathrm{He}$, $^{7}\mathrm{Be}$) reaction has been studied at a laboratory energy of 25.5 MeV for $^{16}\mathrm{O}$ and $^{24}\mathrm{Mg}$ targets and at energies of 25.5, 27.0, and 29.0 MeV for a $^{12}\mathrm{C}$ target. Angular distributions have been measured from ${\ensuremath{\theta}}_{\mathrm{lab}}=15\ifmmode^\circ\else\textdegree\fi{} \mathrm{to} 160\ifmmode^\circ\else\textdegree\fi{}$ for transitions to low-lying states in the residual nuclei. The resulting angular distributions have been analyzed (1) using an exact-finite-range distorted-wave Born-approximation formalism including multistep processes and (2) using a Hauser-Feshbach compound-nuclear model. In general, the shapes of the forward angle ($^{3}\mathrm{He}$, $^{7}\mathrm{Be}$) data are well described by the finite-range distorted-wave Born-approximation and finite-range coupled-channels Born-approximation calculations. The magnitude of the measured transition to the 1.63-MeV (${2}^{+}$) state in $^{20}\mathrm{Ne}$ is approximately 100 times larger than predicted by an ${\mathrm{SU}}_{3}$ theory for a simple one-step transition, and this transition is shown to be an example in which a finite-range coupled-channels Born-approximation analysis is required. Arguments concerning the direct nature of the reaction process populating these states are discussed, and spectroscopic information is extracted.NUCLEAR REACTIONS $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{24}\mathrm{Mg}$($^{3}\mathrm{He}$, $^{7}\mathrm{Be}$), ${E}_{(^{3}\mathrm{He})}=25.5, 27.0, 29.0$ MeV, measured $\ensuremath{\sigma}(E,\ensuremath{\theta})$ Hauser-Feshbach and finite-range CCBA analysis, spectroscopic factors.