Drift Velocity of Electrons in Helium

Abstract

The drift velocity of electrons in helium at 300\ifmmode^\circ\else\textdegree\fi{}K has been measured for $\frac{E}{p}$ values between 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ and 40 volt/cm-mm Hg. The data for $\frac{E}{p}<1$ volt/cm-mm Hg were obtained from measurements of electron transit time in a modernized version of the double shutter tube developed by Bradbury and Nielsen. The data at high $\frac{E}{p}$ were obtained from microwave measurements of the electron density in a positive column of a low-pressure discharge. The measured drift velocities are in good agreement with previous results in the $\frac{E}{p}$ range from ${10}^{\ensuremath{-}1}$ to 3 volt/cm-mm Hg. At $\frac{E}{p}$ less than 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ volt/cm-mm Hg the electrons are essentially in thermal equilibrium with the gas. Margenau's theoretical expression for the drift velocity of electrons in a gas for which the cross section for momentum transfer is independent of electron energy is found to fit the data for $\frac{E}{p}<1$ volt/cm-mm Hg to the accuracy of measurements. The cross section which gives the best fit of the theory to the data is 6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$.