A study of excited levels in Be^8 via the reaction of Li^6(He^3,p)Be^8*

The excited levels of Be^8 in the region of excitation energies between 14 MeV and 17 MeV have been studied by observing the spectrum of protons produced in the reaction Li^6(He^3, p)Be^(8*). Detailed spectrum studies were carried out with a magnetic spectrometer at bombarding energies between 1.6 and 2.8 MeV at 90° in the laboratory. The structure observed consisted of two peaks in the proton spectrum, corresponding to excitation energies of 16.631 ± 0.006 and 16.941 ± 0.008 MeV in B^8. No trace of any other level was found at the excitation energies studied. A narrow level near 16.08 MeV would have been observed if the production cross section had been larger than 0.1 millibarns per steradian. The widths in the center of mass system for the two observed peaks have been determined to be 82 ± 6 and 93 ± 7 keV. Both of these levels were found to decay into alpha particles. The spectrum of alpha particles showed a plateau-like structure corresponding to the breakup of these levels. The detailed shape of the structure is deducible from a kinematic analysis involving energy and momentum conservation, and the observed angular distribution of the outcoming protons. The angular distribution of the protons leading to these levels has been measured at bombarding energies of 2.2, 2.4, and 2.6 MeV. Cross sections were measured at angles between 0 and 150 degrees in the laboratory. The angular distributions of the protons leading to the lower level at 16.63 MeV all show a single forward peak; that for the 16.94 level was not observed with sufficient accuracy to show an unambiguous pattern; there is a suggestion of a dip at forward angles with a maximum near 40 degrees. These angular distributions may be interpreted in terms of a direct mechanism involving the transfer of a neutron and a proton as a single lump. The analysis suggests that the 16.63 level is probably produced by capturing the deuteron into an orbit of zero angular momentum about the Li^6 as a core; the angular distribution of the protons leading to the higher level is compatible with an assumed capture into an orbit of L = 2, but the evidence is generally weaker, so that other possibilities are not strongly ruled out. The interpretation of the data has been carried out using a plane-wave theory of stripping. A brief consideration is given to possible effects of distortions of the plane-wave motion, and the possible effects of the electrical polarizability of the bombarding particles. An explanation of the angular distributions and the magnitudes of the cross sections is offered; it is suggested that these measurements tend to confirm the guess that the level at 16. 63 is the T = 1 analog of the J = 2^+ ground states of Li^8 and B^8. The level at 16.94 should then be T = 0. If one optimistically disregards the weakness of the evidence, the assumed capture into an L = 2 orbit suggests that it may have J = 4^+.