Measurement and prediction of the drift-distance/drift-time relationship of a small jet chamber operated with carbon dioxide/isobutane

Abstract

As part of the preliminary studies for the vertex jet chamber of OPAL a small prototype chamber has been operated using carbon dioxide/isobutane and argon/ethane. A nitrogen laser has been used to produce an ionizing track in the chamber at a distance to the sense-wire plane that could be varied by means of a quartz pentaprism attached to a micrometer. This has enabled the drift-distance/drift-time relationship to be measured with an accuracy of 5–10 μm. The drift-distance/drift-time relationship has also been predicted from calculations of the electric field configuration inside the chamber and of the electron drift-velocity as a function of electric field. Measurement and predictions agree to the level of ∼ 10 μm, thus demonstrating the feasibility of drift-distance/drift-time predictions for the final vertex jet chamber of OPAL, for which detailed laser measurements will not be possible. Corrections to the drift-distance/drift-time relationship for small variations in pressure, temperature and gas composition have also been predicted and shown to be in agreement with experimental measurements. An an alternative to the separate measurement of pressure, temperature and gas composition, it has been demonstrated that a direct measurement of the drift-velocity at one point in the chamber enabled the corrections to the drift-distance/drift-time relationship over the entire chamber volume to be calculated to the ∼10 μm level.