SemidirectT-Forbidden ReactionC12(d,α)B0+,T=110

Abstract

The $T$-forbidden ($d,\alpha$) reaction on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ leading to the $0^{+}$, $T=1\mathrm{}$ state in ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ 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 $E_d=13\mathrm{} \mathrm{to} 15$ MeV with peak cross sections of 120 and 90 μb/sr, respectively. The resonances correspond to states in the compound nucleus ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ at $E_x=21.15\mathrm{}\pm 0.15$ MeV with a width $\Gamma$ of $\approx 1.4$ MeV, and at $E_x=22.70\mathrm{}\pm 0.10$ MeV with a width of $\approx 0.9$ MeV. This is the region of the giant electric dipole resonance. From $E_d=15.5\mathrm{} \mathrm{to} 21$ 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 ($d,\alpha$)-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) $E1\mathrm{}$ Coulomb excitation of the deuteron (polarization of the deuteron in the Coulomb field of the other nucleus) can probably explain the experimental results for $E_d>15.5\mathrm{}$ MeV. However, it appears that these processes cannot account for the magnitude and the resonancelike behavior of the cross section at $E_d=13-15\mathrm{}$ 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.