Measurements have been made of the differential cross sections for the ${\mathrm{O}}^{16}(\alpha ,{\mathrm{Be}}^8){\mathrm{C}}^{12}$ reactions to the ground state and to the 4.43-MeV first excited state of ${\mathrm{C}}^{12}$ at nine bombarding energies between 35.5 and 41.9 MeV. The ${\mathrm{Be}}^8$ nuclei (in their ground state) were identified by detecting the two breakup alpha particles in coincidence, and by imposing the energy and angular requirements for breakup into a narrow cone. The measurements extend, for the most part, from 18° to 100° (c.m.). The angular distributions display oscillatory behavior with typical separations between peaks of 15° to 20° (c.m.). Strong fluctuations in differential cross sections were observed with changes in incident energy. The ground-state and 4.43-MeV-state yields are uncorrelated, except perhaps at 35.5 MeV where both yields are particularly large. With the exclusion of the 35.5-MeV data, the cross sections integrated from 18° to 100° (c.m.) have mean values of 0.6 mb for the ground-state reaction, and 1.0 mb for the 4.43-MeV-state reaction. The existence of the observed fluctuations refutes the original expectation that simple direct interaction processes would account for the ${\mathrm{O}}^{16}(\alpha ,{\mathrm{Be}}^8){\mathrm{C}}^{12}$ reaction. The results can be qualitatively understood in terms of the statistical theory of compound nuclear reactions, although a substantial nonstatistical component cannot be excluded. The average width of the compound nuclear states, given by the application of the statistical viewpoint to the experimental results, is 1.1 MeV.

• Received 15 February 1965

DOI:https://doi.org/10.1103/PhysRev.138.B1394