Observation of a Positron Mobility Threshold in Gaseous Helium

The lifetime spectra of positrons subjected to a pulsed electric field while slowing down in low-temperature gaseous helium reveal a mobility threshold $E_c^{}{}_{}{}^{+}$, below which the positrons exhibit no observable mobility. Energy-loss rates for collisions of a positron with a single helium atom provide an estimate of $E_c^{}{}_{}{}^{+}=15\mathrm{}\pm 3$ meV and an upper limit on the width of this threshold of 1.5 meV. Possible connections between $E_c^{}{}_{}{}^{+}$ and Anderson localization are discussed.