The presence of a strong discontinuity in the free $\alpha -\alpha$ scattering amplitude at 40 MeV is used as a probe for the identification of $\alpha$ clusters in nuclei. The ${}_{}{}^6{}_{}{}^{}\mathrm{Li}(\alpha ,2\mathrm{}\alpha )$ reaction is investigated at 37.5 and 43.5 MeV with the deuteron as spectator in order to produce quasifree scattering conditions. The distributions measured at these two energies are found to be very different in magnitude as well as in structure, and this phenomenon is attributed to the presence of $\alpha -\alpha$ scattering in the $\alpha -{}_{}{}^6{}_{}{}^{}\mathrm{Li}$ collision. Calculations based on the plane-wave impulse approximation are unable to reproduce the distributions even qualitatively. A calculation based on the graph formalism which takes into account the exact phase-shift amplitude of the free $\alpha -\alpha$ collision fits the two distributions with the same set of parameters. One may conclude that these observations are in favor of the existence of $\alpha$ particles inside the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ nucleus.

• Received 12 October 1970

DOI:https://doi.org/10.1103/PhysRevC.3.2085