$\alpha +\alpha$ scattering has a long history since the first experiment by Rutherford and Chadwick in 1927 and has been studied thoroughly experimentally and theoretically. However, $\alpha +\alpha$ scattering has never been paid attention from the viewpoint of refractive scattering. I have successfully analyzed the experimental angular distributions in $\alpha +\alpha$ scattering systematically over a wide range of incident energies $E_L=53.4–280\mathrm{MeV}$ using a phenomenological optical model with a deep real potential. The existence of a farside-dominant quasinuclear rainbow with no well-defined rainbow angle and no supernumerary bow in the lit side followed by the shadow, which is not a genuine rainbow but a refractive scattering from a marginally small droplet at high energies, is found for the first time in $\alpha +\alpha$ scattering. The refraction due to the deep potentials with an attractive core at short distances are discussed from the viewpoint of the Luneburg. The deep vs shallow problem of the potential and the nuclear rainbow scattering in inelastic channels are also discussed.

• Received 11 December 2023
• Accepted 13 March 2024

DOI:https://doi.org/10.1103/PhysRevC.109.034618