Studies on Production of Impurities in Gaseous Detectors Operated with Fluorinated Gases

Large Hadron Collider (LHC) experiments at CERN involve various detectors which are operated with fluorinated gases. Resistive Plate Chambers (RPCs) and Cathode Strip Chambers (CSCs) can be given as such gaseous detectors. The use of fluorinated gases diminishes ageing effects for CSCs, whilst enhancing RPC efficiency, but causing performance issues over long periods. The operation of such detectors at LHC conditions alters the chemical composition of the gas mixtures in use, thus changing detector performances. A laboratory setup was prepared at Building 904 to allow for Ion Selective Electrode (ISE) measurements to be carried out with a simple straw tube detector setup, to analyse the behaviour of CSC gas mixtures at different conditions and determine the applicability of ISE measurements to analyse CSC detectors. Measurements quantifying the fluoride ion output of the straw and environmental conditions were carried out. These were successful in characterising impurity production in standard CSC gas mixtures. The findings were used to analyse the link between fluoride ion production rate and different gas mixtures in use. The studies allowed for initial fluoride ion production rates to be calculated at a flow rate of (0.5 ± 0.05)l/h, with detector currents of (0.350 ± 0.001)μA, and a 235MBq 90Sr beta source. The studies showed that carbon tetrafluoride percentages of 10% and 7% did not have significantly different fluoride production rates.