Long term stability and uniformity studies of MDT chambers in the H8 2003 system test

The drift properties of six-barrel chambers installed on the H8 beam line have been studied. The chambers have been kept at constant gas fluxes for a period of more than one month and the data collected during this period of the 2003 run have been analyzed. The drift properties of each tube have been evaluated by means of the measurement of the maximum drift time. Particular emphasis is given to the study of the drift properties of tubes that are in different positions in the on-chamber gas distribution series. A clear dependence of the drift properties on the series position of the tubes has been observed. The dependence of this effect on time, temperature and position along the tube has been investigated. An attempt to give a quantitative explanation of the effect based on the permeability of the MDT end-plugs to water is performed. An approximated estimate of the impact of the effect on single tube resolution is derived.