Differential cross sections for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{12}$C, ${}_{}{}^8{}_{}{}^{}\to 2\mathit{\alpha }$${)}^{16}$O reaction have been measured at eight incident energies in the 2.43≤$E_{\mathrm{c}.\mathrm{m}.\mathrm{}\mathrm{\le }5.24}$-MeV range, detecting α particles in 200-μm thick Makrofol polycarbonate foils, which were subsequently carefully etched and scanned under microscope for the ${}_{}{}^8{}_{}{}^{}$Be→2α group and the α groups from the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{12}$C, α${)}^{20}$Ne reaction. The ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ channel cross section, ranging between 4×${10}^{\mathrm{-}4}$ μb and 280 μb, is smaller for $E_{\mathrm{c}.\mathrm{m}.\mathrm{}\ge 3.91}$ MeV, but larger for $E_{\mathrm{c}.\mathrm{m}.\mathrm{}\mathrm{\le }3.13}$ MeV than the ${\alpha }_0$${+}^{20}$${\mathrm{Ne}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}$ channel cross section. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{12}$C, ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$${)}^{16}$O cross section is well reproduced by the distorted-wave Born approximation calculation of a direct α-transfer reaction down to $E_{\mathrm{c}.\mathrm{m}.\mathrm{}=3.91\mathrm{}}$ MeV. In the 3.13≥$E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ ≥2.43-MeV range, however, the measurements exceed the distorted-wave Born approximation prediction by one to three orders of magnitude. It is suggested that the formation of ${}_{}{}^{12}{}_{}{}^{}\mathrm{-}_{}^{12}{}_{}{}^{}$C dinuclear states, favoring an α transfer, becomes a predominant process at incident energies $E_{\mathrm{c}.\mathrm{m}.\mathrm{}\mathrm{\le }3.13}$ MeV.

• Received 11 October 1988

DOI:https://doi.org/10.1103/PhysRevC.39.1326