Role of multi-step processes in ¹⁶O(¹¹B,¹²C)¹⁵N at 41.25 MeV

Abstract:

The ¹⁶O(¹¹B,¹²C)¹⁵N reaction at 41.25 MeV has been investigated using the kinematical coincidence method. Polarization tensors t₂₀ and t₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured at center-of-mass angles (Θ_c.m.) between 48^° and 62^° by analyzing the energy spectrum of ¹²C[2⁺ ₁] modulated by the effect of γ-ray emission. The cross-sections of the transfer reactions leading to the ¹²C[g.s.]+¹⁵N[g.s.], ¹²C[2⁺ ₁]+¹⁵N[g.s.] and ¹²C[g.s.]+¹⁵N[3/2^- ₁] final states have also been measured in the range 48^°≤Θ_c.m.≤ 120^°. The polarization tensor terms of ¹²C[2⁺ ₁] largely deviating from zero have been observed, contrary to the prediction by the distorted-wave Born approximation (DWBA). The one-step DWBA calculation also fails in describing the transfer reaction cross-sections. It is shown that the coupled channel model calculation including excitations and reorientations in ¹¹B and ¹²C satisfactorily reproduces both the tensor terms and the cross-sections of the transfer reactions. The multi-step processes passing through the excited states of ¹¹B are found to significantly contribute to the reaction.