Proton Transfer to Specific Final States in the ReactionAl27(O16,N15)Si28near the Coulomb Barrier

Abstract

The angular distribution of ${\mathrm{N}}^{15}$ particles, resulting from the transfer reaction ${\mathrm{Al}}^{27}$ (${\mathrm{O}}^{16}$,${\mathrm{N}}^{15}$) ${\mathrm{Si}}^{28}$, was determined at three incident energies, 28.5, 30.0, and 36.0 MeV. The ${\mathrm{O}}^{16}$ ions were accelerated in the Oak Ridge Tandem Van de Graaff. Two silicon-surface barrier counters were placed at the proper angles to detect the ${\mathrm{N}}^{15}$ and ${\mathrm{Si}}^{28}$ particles in coincidence. A proportional counter imposed the further restriction that particles observed in the defining counter be nitrogen nuclei. Angular distributions were determined for two particular final-state transfers: (1) leaving both residual nuclei in their ground states, and (2) leaving the ${\mathrm{Si}}^{28}$ in its first excited state. Within experimental error the two distributions for a given bombarding energy were found to peak at approximately the same center-of-mass angle. By integrating the differential cross sections, the total reaction cross sections, in order of increasing energy, were determined to be: (1) 0.88, 1.35, and 1.37 mb for the ground-state transfers, and (2) 0.96, 1.72, and 2.72 mb for the excited-state transfers. The angular distributions were compared with the predictions of the tunneling theory of Breit bearing in mind that the theory, as formulated, is applicable only to the transfer of neutrons, not protons, and only at incident energies below the Coulomb barrier. As expected, the agreement was found to be better at 28.5 and 30.0 MeV than at 36.0 MeV.