Microscopic Gas-Liquid-Like Phase Transition around the Positron in Helium Gases

The clustering of atoms around slow positrons in helium gases is experimentally studied and the results are interpreted in terms of liquid drop formation. The critical temperature and pressure of the droplet phase are found to be 6.6 K, 5.4 atm in ${}_{}{}^2{}_{}{}^{}\mathrm{He}$ and 8.4 K, 7.6 atm in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The critical densities coincide with those of the helium liquids.