B. Mandelli

The gas systems for the LHC experiments

Over the five experiments (ALICE, ATLAS, CMS, LHCb and TOTEM) taking data at the CERN Large Hadron Collider (LHC) more than 28 gas systems are delivering the proper gas mixture to the corresponding detectors. They are complex systems that extend over several hundred meters and have to ensure an extremely high reliability in terms of stability and quality of the gas mixture delivered to the detectors. In fact, the gas mixture is the sensitive medium and a correct and stable composition is basic requirements for good and safe long term operation. The present contribution describes the design philosophy focusing the attention on the main functional modules. The reliability over the past years is also discussed.

RPC performances and gas quality in a closed loop gas system for the new purifiers configuration at LHC experiments

In the ATLAS and CMS experiments the RPC detector covers a large surface of about 4000 m2 equivalent to 14 m3 of gas volume for each system. RPCs are operated using a C2H2F4 (R134a) based humidified gas mixture. A flow of the order of one volume exchange per hour is needed for the detector operation. These characteristics make the closed-loop circulation unavoidable. Nowadays the gas systems are operated with a 90–95% re-circulation factor. Results from tests performed over the past few years have shown how the molecules in the gas mixture are broken up under the action of the high electric field and the high radiation background during LHC operation. Several RPCs were operated at the CERN Gamma Irradiation Facility in a high radiation environment in order to observe the production of typical impurities and to find an optimal purifiers configuration for their absorption. During the test, the detector performances were monitored in terms of current stability and HPL resistivity.

Development of a common gas analysis approach for the gas systems of all the experiments at the CERN Large Hadron Collider

Over the five experiments (ALICE, ATLAS, CMS, LHCb and TOTEM) taking data at the CERN Large Hadron Collider (LHC) more than 28 gas systems are delivering the proper gas mixture to the corresponding detectors. They are complex systems that extend over several hundred meters and have to ensure an extremely high reliability in terms of stability and quality of the gas mixture delivered to the detectors. In fact, the gas mixture is the sensitive media and a correct and stable composition is basic requirements for good and safe long term operation. In this context the monitoring of the gas mixture becomes of fundamental relevance. O2 and H2O analysis racks are already available on all the experimental sites. They provide a fast and simple quality check. Gas chromatographic stations are installed on two experiments and already in the past year they revealed to be very useful. In the present contribution, an overview of the analysis methods and few practical examples are described.

Systematic study of RPC performances in polluted or varying gas mixture compositions: An online monitor system for the RPC gas mixture at LHC

A correct gas mixture composition is fundamental for proper and safe operation of the Resistive Plate Chamber large detector systems. At long term, small changes in the gas mixture composition can alter the RPC performance affecting the data quality in the ALICE, ATLAS and CMS experiments at CERN. A constant monitoring of the gas mixture quality injected in the RPC systems would possibly avoid this kind of problems. A systematic study has been performed to understand the RPC performances with slightly different gas mixture compositions and in presence of common impurities. The systematic analysis of several RPC parameters in different gas mixtures allows the quick identification of variations in the mixture. A set-up for the online monitoring of the RPC gas mixture in the LHC experiment is also proposed.

Resistive plate chamber operation with new environmentally friendly gases

Resistive Plate Chamber (RPC) detectors are widely employed in the muon trigger systems of three experiments at the Large Hadron Collider (LHC) thanks to an excellent time resolution. A gas mixture composed of C₂H₂F₄, iC₄H₁₀ and SF₆ is used for LHC RPCs operation in avalanche mode. C₂H₂F₄ and SF₆ have a Global Warming Potential (GWP) of 1430 and 23900 respectively, classifying them as greenhouse gases. The search of new environmentally friendly gas mixtures is advisable for reducing greenhouse gas emissions, operational costs as well as to optimize RPC performance and possible detector aging issues. Two eco-friendly candidates have been identified for substitution of C₂H₂F₄: R1234yf and R1234ze with a GWP of 4 and 6, respectively. A dedicated experimental set-up has been implemented to study single-gap RPC performance in terms of avalanche and streamer operation together with the evaluation of the quenching and electronegative capacities of the selected environmentally friendly Freon. Several new gas mixtures making use of only very low GWP gases have been tested. The first tests confirm that the simple replacement of C₂H₂F₄ and SF₆ with the new Freon is not possible and the addition of more reactive gases is necessary to achieve the required performance at LHC. RPCs have been successfully operated in streamer mode in a mixture of R1234yf, Ar and iC₄H₁₀.

Studies of IBL wire bonds operation in an ATLAS-like magnetic field and evaluation of different protection strategies

At the Large Hadron Collider (LHC) experiments, most of silicon detectors use wire bonds to connect front-end chips and sensors to circuit boards for the data and service transmissions. These wire bonds are operated in strong magnetic field environments and if time-varying currents pass through them with frequencies close to their mechanical resonance frequency, strong resonant oscillations may occur. Under certain conditions, this effect can lead to fatigue stress and eventually breakage of wire bonds. Systematic studies have been conducted to analyse the effects of resonance vibration on wire bonds. In particular, the case of the Insertable B-Layer (IBL) detector, the new innermost layer of the ATLAS Pixel Detector, has been reviewed. An experimental set-up has been built to simulate as much as possible the operation conditions of IBL wire bonds in the ATLAS magnetic field. The results provide useful information for the comprehension of the IBL wire bonds behavior. The dangerous resonance frequencies have been identified experimentally for different wire bond lengths. The resonance frequency amplitudes have been characterized in terms of several parameters, like wire length, wire orientation angle with respect to B-field and current amplitude. Several fatigue studies have been performed with simulations and laboratory tests. It has been demonstrated that in well-defined conditions, as for example with high currents, the wires can get irreparably damaged after few oscillation cycles and they can break. Two types of wire bond protections have been considered: the classical encapsulation of the wire feet and the coating of the whole wire. The results reveal that these methods minimize the oscillation amplitude reducing the possibility of damaging or breaking the wire bonds. For the IBL detector a Fixed Frequency Trigger Veto has been implemented for excluding the potentially dangerous frequencies identified in these studies.

Gas Systems for Particle Detectors at the LHC Experiments: Overview and Perspectives

Over the five experiments (ALICE, ATLAS, CMS, LHCb and TOTEM) taking data at the CERN Large Hadron Collider (LHC) 30 gas systems are delivering the proper gas mixture to the corresponding detectors. They are complex systems that extend over several hundred meters and have to ensure an extremely high reliability in terms of stability and quality of the gas mixture delivered to the detectors. In fact, the gas mixture is the sensitive medium and a correct and stable composition is a basic requirement for good and safe long-term operation. The present contribution describes the design philosophy focusing the attention on the main functional modules. The reliability over the past years is also discussed.

Gas Mixture Monitoring Techniques for the LHC Detector Muon Systems

At the LHC experiments the Muon Systems are equipped with different types of gaseous detectors that will need to assure high performance until the end of the LHC run. One of the key parameters for good and safe long-term detector operation is the gas mixture composition and quality. Indeed a wrong gas mixture composition can decrease the detector performance or cause aging effects and irremediable damages. It is therefore a fundamental requirement to verify and monitor the detector gas mixture quality.

Characterization of RPC operation with new environmental friendly mixtures for LHC application and beyond

The large muon trigger systems based on Resistive Plate Chambers (RPC) at the LHC experiments are currently operated with R134a based mixture. Unfortunately R134a is considered a greenhouse gas with high impact on the enviroment and therefore will be subject to regulations aiming in strongly reducing the available quantity on the market. The immediat effects might be instability on the price and incertitude in the product availability. Alternative gases (HFO-1234yf and HFO-1234ze) have been already identified by industry for specific applications as replacement of R134a. Moreover, HFCs similar to the R134a but with lower global warming potential (GWP) are already available (HFC-245fa, HFC-32, HFC-152a). The present contribution describes the results obtained with RPCs operated with new enviromemtal friendly gases. A particular attention has been addressed to the possibility of maintening the current operation conditions (i.e. currently used applied voltage and front-end electronics) in order to be able to use a new mixture for RPC systems even where the common infrastructure (i.e. high voltage and detector components) cannot be replaced for operation at higher applied voltages.

Strategies for reducing the environmental impact of gaseous detector operation at the CERN-LHC experiments

Over the five experiments (ALICE, ATLAS, CMS, LHCb and TOTEM) taking data at the CERN Large Hadron Collider (LHC) more than 28 gas systems are delivering the proper gas mixture to the corresponding detectors. In some cases the use of expensive and/or greenhouse gases cannot be avoided because of physics requirements that impose certain choice on the gas mixture. Typical example is the use of Freon like R134a, CF 4 and SF 6 . The present contribution describes the current status with the new strategies applied to the LHC gas system in order to reduce operational cost and greenhouse gases emission.