C12(C12, α)Ne20excitation functions and angular distributions

Abstract

Excitation functions have been measured at a laboratory angle of 5° over an incident energy range of 18-25.5 MeV (c.m.) for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, \alpha )$ reaction leading to about 30 levels in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ ($E_x=0-20\mathrm{}$ MeV). Angular distributions were measured at five energies. An Ericson fluctuation analysis yields the result that most levels are populated by a large nonstatistical reaction component. A significant compound-nuclear component is also present, with correlation widths on the order of 150-250 keV (c.m.). The energy-averaged cross sections have been compared with statistical compound-nuclear (Hauser-Feshbach) calculations. This comparison indicates the presence of a strong direct-reaction mechanism, especially for the members of the proposed 8p-4h band (at 7.196, 7.834, 9.040, and 12.16 MeV). A pronounced minimum in the excitation functions is evident at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=19.2\mathrm{}$ MeV, near the recently reported strong resonance in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, p){}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ channel. Other correlated effects also appear to be present at specific energies, but the cross correlations, calculated for the entire energy range, are largely statistical.