Gas monitoring in RPC by means of non-invasive plasma coated POF sensors

Resistive Plate Counters (RPC) are employed as muon detectors in many high-rate high-energy physics experiments, such as the Compact Muon Solenoid (CMS) experiment currently under way in the Large Hadron Collider (LHC) accelerator at the European Center for Nuclear Research (CERN). A gas mixture containing C2H2F4, i-C4H10 and SF6 is recirculated inside the RPCs during their use and subjected to degradation due to the production of fluoride ions which limits the sensitivity of the RPCs. This paper describes a new sensor that is able to detect low concentrations of fluoride ions in gas mixtures. The sensor is made of a plastic optic fiber (POF) which is made sensitive to F- gaseous ions by means of a thin layer of a glass-like material, deposited via plasma onto the fiber core. The F- ions attack the glass-like film and alter the transmission capability of the fiber so that the detection simply requires a LED and a photodiode. The sensor exploits a cumulative response which makes it suitable for direct estimation of the total exposure to the F- ions, thus providing a tool that can be used to tune the maintenance of the gas filters. The glass-like film is deposited by means of plasma enhanced chemical vapor deposition (PECVD) of organosilicons monomers, which allows the deposition to be performed a low temperature in order to avoid damaging the fiber core.