Investigation of the mechanism of ρ-induced four-nucleon pick-up reactions

Abstract

In the first part of this paper, the α-transfer character of the (τ, 7Be) reaction which is shown to be direct is examined in the light of two particular experimental features: (i) the variation of the cross section with the mass of the target nucleus, and (ii) the hindrance of ΔT ≠ 0 or ΔS ≠ 0 transitions. The experimental evidence indicates that the dominant mechanism in the (τ, 7Be) reaction to low-lying states is a direct α-transfer. However, in the second part of the paper we investigate the possibility of a mechanism in which the two protons and two neutrons transferred are in a T = 1 state through an experimental study of the analogue process, the transfer of a 4H-like fragment. The corresponding reaction, (τ, 7Li), is found to proceed through a direct mechanism at forward angles. It is concluded, for the 19F(τ, 7Be)15N ground state transition, that the magnitude of the T = 1 transfer amplitude is approximately √0.2 times the total transition amplitude. A simple estimate of the (τ, 7Li) and (ρ, 7Be) reaction strengths. D0, suggests that 2p+2n, T = 1 clustering in light and medium nuclei is less probable than T = 0 α-clustering by almost two orders of magnitude.