Confirmation of Positron Mobility Edge in Gaseous Helium by Monte Carlo Simulation

Positron lifetime spectra in helium gas, subjected to a uniform dc electric field, are calculated by a Monte Carlo simulation procedure which is based on an earlier scheme developed by Farazdel and Epstein. These calculations clearly show that introduction of a positron mobility edge $E_c^{}{}_{}{}^{+}$ alone produces a striking improvement in the agreement between theory and experiment. By visual inspection it is estimated that $E_c^{}{}_{}{}^{+}={10}_{-2\mathrm{}}^{+5\mathrm{}}$ meV at an electric field of 52 V/cm.