Scattering of Alpha Particles from Helium

Abstract

Excitation curves and angular distributions for the scattering of $\alpha$ particles from helium have been obtained for bombarding energies of 3 to 6 Mev, using a differentially pumped gas scattering chamber and a singly charged $\alpha$-particle beam from the Rice Institute Van de Graaff accelerator. A phase shift analysis of the data, yielding phase shifts accurate to about 2°, reveals that only $S$- and $D$-wave scattering occurs and shows that the well-known excited state at 2.9 Mev in ${\mathrm{Be}}^8$ is a $D$ state. The scattering data taken at the Department of Terrestrial Magnetism and the Universities of Illinois and Indiana are compared to the Rice Institute data by using the dispersion theory and the Landau $K$-function formalism developed for proton-proton scattering. The dispersion theory, applied to the Rice data, allows a rough estimate of the $D$-state level parameters, while the $K$ function, applied to the Rice and D.T.M. data, is used to deduce a ground state width for ${\mathrm{Be}}^8$ of 4.5±3 ev (c.m.). For excitation energies below 12 Mev, the scattering data are consistent with a description of the ${\mathrm{Be}}^8$ nucleus as an interaction of two $\alpha$-particles. Certain qualitative features of the potential of interaction of such a two-body configuration are discussed.