DRIFT VELOCITY OF ELECTRONS IN HELIUM-BASED GAS-MIXTURES MEASURED WITH A UV LASER

For the central tracking chamber at the KEK B-factory, the use of a lo w Z gas in the drift chamber is preferred. We have measured the drift velocity of electrons in helium-based gas mixtures using a pulsed UV l aser beam. In this paper, we describe the results of our precision mea surement of drift velocities using various mixtures of helium and etha ne. In addition, the effect of water vapor contamination on the drift velocities is also studied.