SEMIDIRECT T-FORBIDDEN REACTION 12C(D,ALPHA)10BO+,T=1

The T-forbidden (d,α) reaction on C12 leading to the 0+, T=1 state in B10 has been investigated at deuteron bombarding energies from 13 to 21 MeV. The shape of the angular distributions is practically energy-independent. The angular distributions are characteristic of a direct-reaction mechanism, in agreement with the results obtained by Meyer-Schützmeister, v. Ehrenstein, and Allas for deuteron energies from 11 to 13 MeV. The excitation function shows two pronounced resonances in the range from Ed=13 to 15 MeV with peak cross sections of 120 and 90 μb/sr, respectively. The resonances correspond to states in the compound nucleus N14 at Ex=21.15±0.15 MeV with a width Γ of 1.4 MeV, and at Ex=22.70±0.10 MeV with a width of 0.9 MeV. This is the region of the giant electric dipole resonance. From Ed=15.5 to 21 MeV, the cross section is small and the excitation function is practically energy-independent. The cross section in the peak of the angular distribution is about 5 μb/sr. Thus the reaction exhibits a contradictory behavior, having both direct- and compound-nucleus features. The reaction mechanism is discussed in terms of a two-step process consisting of a T-violating step and a (d,α)-like reaction. For the T-violating step, we have considered virtual Coulomb excitation and the preferential spin-flip process suggested recently by Noble for deuteron-induced reactions. A consideration of the characteristics and estimated magnitudes of the various effects suggests that the spin-flip process or (virtual) E1 Coulomb excitation of the deuteron (polarization of the deuteron in the Coulomb field of the other nucleus) can probably explain the experimental results for Ed>15.5 MeV. However, it appears that these processes cannot account for the magnitude and the resonancelike behavior of the cross section at Ed=13-15 MeV. A mechanism is suggested which postulates the existence of certain clusterlike, quasibound two-particle-two-hole states in the region of the giant resonance. The existence of such states has been suggested by Gillet, Melkanoff, and Raynal in a study of the fine structure of the giant resonance. The proposed reaction would proceed by an induced spin-flip in the deuteron at the corresponding bombarding energies. © 1968 The American Physical Society.