A qualitative method to determine the loss of efficiency in the DELPHI Barrel RICH MWPCs due to ageing

Abstract

We describe a simple method to survey effects of ageing in the multiwire proportional chambers of the DELPHI Barrel Ring Imaging CHerenkov (RICH) counters. These chambers detect single photoelectrons created by Cherenkov light in a photosensitive gas mixture. They receive as well the much larger amount of ionisation electrons created by the charged particles. The method essentially consists of a comparison of the plateau curves measured after each year's running period in order to detect changes in their characteristics as a function of exposure.