Experimental Observation of a Predicted Exchange Resonance inHe3+He4Elastic Scattering

Resonating-group calculations predict a backward-peaked, exchange resonance in the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ + ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ elastic-scattering cross section. We have measured the energy dependence of the elastic-scattering differential cross section at 173.2° (c.m.) and have observed a resonance structure which has many of the predicted characteristics.