An overview of RPCs at the LHC startup

This issue of the RPC workshop occurs in a particular historical moment which is characterized by a number of RPC systems already in data taking or in phase of advanced commissioning. In particular the startup of the Cern LHC, foreseen this year, will be a crucial test for the trigger and the time-of-flight RPC systems that have been developed for both trigger and time-of-flight by most of the LHC experiments. This offers the opportunity to evaluate the results achieved by very large systems. This introductory talk will stress some of the main achievements and problems of the last two years. The choice of the topics is somewhat arbitrary and apologies are due for the relevant results not mentioned here. & 2009 Elsevier B.V. All rights reserved. 1. The impact of the working gas for very large systems The feasibility of extremely large RPC systems is strongly constrained by the amount of gas needed for keeping them working in the long term. The gas represents indeed the most relevant maintenance and operation cost. As an example the ATLAS and CMS RPC detectors at LHC have active volumes of about 15m3 and require some 60kg of gas each just to be filled. The need for efficient gas purification/recirculation systems is therefore mandatory in view of 10 years of operation at LHC. The most relevant gas impurities for the RPCs, mostly fluorine compounds, are those produced by the gas decomposition under electrical discharge. Their production rate increases with the detector counting rate and is proportional to the operating current. The contamination by small amounts of air, which may happen for gaseous detectors, although also important for the chamber performance, is comparatively less dangerous for the long-term detector health. The LHC RPCs that are expected to operate at very high rate at full LHC luminosity require gas loops capable of very large recirculation flows with modest need of fresh gas. The Atlas and CMS RPC trigger detectors are presently operating in closed loop at a recirculation rate of about 0.4volumes/h with a fresh gas flow ranging between 5% and 10%. The corresponding chamber performance for the correct working of the closed loop is substantially indistinguishable from the open flowcase. This confirms a result already obtained in the ageing test carried out at the Gamma Irradiation Facility (GIF) of CERN [1,2]. It should be stressed, however, that this result refers at present ll rights reserved. , University of Rome ‘‘Tor , Italy. to cosmic ray tests. In the presence of interacting proton beams the operating current is expected to be much larger and the amount of impurities to be removed by the purification system will be correspondingly higher. The main purpose of the closed loop tests for LHC in the next future will be therefore to establish the lower limit for the flow of fresh gas required in both cosmic ray and beam operation. The impact of the gas is even more relevant for the next generation RPC systems, like the one foreseen for the India-based Neutrino Observatory (INO) which is designed to have an active volume of about 200m3 and will require more than 500kg of gas for a single filling. This makes the efficiency of the gas recirculation/purification systems a central point for future developments of the detector. It should be stressed, however, that gas closed loops conceived for the LHC experiments are complex and sophisticated systems that could hardly be used for detectors operating in high altitude or underground laboratories which require simple and low maintenance devices. A basic consideration in order to design a device of this type is that the most aggressive pollutants of the RPC gas are fluorine compounds like HF produced by the decomposition of the C2H2F4 molecule under electrical discharge. These impurities are easily soluble in water and may be removed by bubbling the exhaust gas in water. A feasibility test based on this concept has been carried out with cosmic rays over a system of 12 chambers of 3:5m2 of the type used for the Argo-YBJ detector [3]. The chambers are filled with the gas mixture TFE/A/iso-C4H10 1⁄4 75=15=10 and operated in streamer mode. The exhaust gas is pumped in the bottom of a water filled steel cylinder, and after bubbling in the water is sent again to the chambers. The chambers were kept in closed loop for 19 days with the addition of 2% (1.8 l) fresh gas per day. All during the test the following parameters were monitored: counting rate, operating current and oxygen contamination. The operating