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The lifetime spectra of positrons subjected to a pulsed electric field while slowing down in low-temperature gaseous helium reveal a mobility threshold E₂^+, 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 E₂^+=153 meV and an upper limit on the width of this threshold of 1. 5 meV. Possible connections between E₂^+ and Anderson localization are discussed.
Tawel et al. (Mon,) studied this question.