The lifetime spectra of positrons subjected to a pulsed electric field while slowing down in low-temperature gaseous helium reveal a mobility threshold Ec⁺, 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 Ec⁺=15±3 meV and an upper limit on the width of this threshold of 1.5 meV. Possible connections between Ec⁺ and Anderson localization are discussed.
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Tawel et al. (1986) studied this question.
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