Measurement of Helium-Helium Scattering Cross Section at Low Temperatures

We have measured the${\mathrm{He}}^4$-${\mathrm{He}}^4$ scattering cross section at 2.1 °K by an atomic-beam method. The value found for the averaged total effective cross section ${\overline{Q}}_{\mathrm{eff}}$ with both beam and scattering gas at 2.1 °K is 144 ${\mathrm{\AA }}^2$, with an estimated accuracy of 12%. We have also measured the total effective cross section as a function of beam velocity, $Q_{\mathrm{eff}}\mathrm{}\left(v\right)\mathrm{}$, with $v$ ranging from 70 m/sec to 210 m/sec. These measurements were made with the scattering gas at 2.1 °K.