Mobility of positrons in polyethylene

We have measured the positron drift velocity versus electric field in polyethylene by observing the Doppler shift of the annihilation photons. We find that the average positron mobility computed using the internal electric field (dielectric constant =2.26) is μ=10.3±1.5 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{\mathrm{-}1}$ ${\sec }^{\mathrm{-}1}$. This value agrees with the measurement of Brandt and Mourino [Bull. Am. Phys. Soc. 24, 72 (1979)], but disagrees with that of MacKenzie and Ghorayshi [Solid State Commun. 55, 125 (1985)], possibly indicating a sensitivity to the precise composition of the sample.