DRIFT VELOCITY IN N-PENTANE MIXTURES AND ITS INFLUENCE ON TIMING PROPERTIES OF THIN GAP CHAMBERS

We report on the measurements of the electron drift velocity and longi tudinal diffusion in n-pentane gas mixtures and on the effect of the d rift velocity on the timing properties of Thin Gap Chambers. Gas mixtu res of n-pentane-CO2, n-pentane-CO2-CH4, and n-pentane-CO2-CF4 were in vestigated. An increase of the drift velocity has been observed with m ixtures containing CF4, while still maintaining the very high, saturat ed gain, typical for these chambers. The overall improvement of TGC ti ming properties is such that 99% detection efficiency can be reached w ithin a 20 ns gate. A simulation of the chamber timing properties usin g the measured drift velocities, reproduces well the measured data and can be used to predict chamber performance as a function of geometry and gas mixture. (C) 1998 Elsevier Science B.V. All rights reserved.