Gamma-ray yields from 12C + 13C reactions near and below the coulomb barrier

Abstract

The γ-ray yields from low-lying transitions in heavy residual nuclei produced in the 12C+ 13C reaction have been measured from E c.m. = 3.1 to 11.9 MeV using a Ge(Li) detector. Total cross sections for compound nucleus formation were deduced from the experimental data with the aid of the Hauser-Feshbach model. Several independent checks on this procedure are described. These tests verify the assumptions made in the analyses of this reaction and suggest that the deduced cross sections have an absolute uncertainty of ±30 %. The present experimental results for the 12C+ 13C reaction are qualitatively very different from those for the 12C+ 12C reaction and do not provide any striking evidence for either broad singleparticle resonances in the total reaction cross section or for narrow non-statistical (quasimolecular) resonances in summed cross sections for proton and for α-particle emission to bound states of 24Na and 21Ne, respectively. The predictions of several optical models employing attractive nuclear potentials are compared to the data. None is successful in reproducing the measured cross sections over the entire range of bombarding energy. The predictions at low energies depend sensitively on the shape of the potential a few fm inside the region of the nuclear surface. A narrow, rapidly varying energy dependence of the γ-ray yields is observed, with a peak-to-valley ratio of typically 1.1. However, a statistical analysis shows that these fluctuations, and those observed in recent charged particle measurements of α-particle yields, are reasonably consistent with those expected from the formation and decay of strongly overlapping levels in the compound nucleus. Finally, several observations are made on the validity of certain approximations often made in statistical analyses of heavy-ion reactions.