Positron Self-Trapping inHe4

Positron-annihilation experiments in helium gas have shown anomalies in annihilation rate near the gas-liquid critical point which indicate the formation of positron self-trapped states in helium droplets. We present theory which appears to confirm the existence of self-trapped states and accounts quantitatively for the positron-annihilation rate in the range of densities and temperatures over which these states are stable. The droplets have a maximum density 2.4×${10}^{22}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and are typically 15 to 25 Å in radius.