CERN Accelerating science

R&D on gas usage in the LHC particle detection systems

A wide range of gas mixtures is used for the operation of different gaseous detectors for particle physics research. Among them are gases like C2H2F4 (R134a), CF4 (R14), C4F10 (R610) and SF6, which are used because they allow to achieve specific detector performance that are necessary for data taking at the LHC experiments (i.e. stability, long term performance, time resolution, rate capability, etc.). Such gases are environmentally unfriendly, as they contribute to the greenhouse effect, and currently subject to a phase down policy that started to affect the market with price increase and, in the long term, may cause a decrease in their availability.  

With the idea of preparing the very long-term operation, the EP-DT Gas Team, the CERN Environmental Protection Steering board (CEPS) and the LHC experiments have elaborated a strategy based on several action lines.

Gas consumption at the LHC experiments is already reduced by operating all particle detector systems using such gases with gas recirculation plants, i.e. systems where the return mixture from the detectors is collected, cleaned and then re-used. In addition, the first identified research line focuses the attention on the optimization of current technologies allowing to improve flow and pressure stability beyond original requirements to cope with new detectors needs. For example, during Run 2 the mixture recirculation rate of the RPC detector systems was limited to 85-90% due to the presence of leaks at the detector level. Despite the difficult challenge, LS2 will give a unique chance to repair as many leaks as possible and to optimize the gas systems.

The second research line is based on the development of systems able to collect the used gas mixture and to extract detector gases for their re-use. Recuperation systems permit to avoid venting to atmosphere the gas injected into recirculation system. Systems allowing the recuperation of different gases have been developed in the past for several detectors: CMS-CSC (CF4), ATLAS-TGC (nC5H12), LHCb-RICH1 (C4F10) and LHCb-RICH2 (CF4). R&D studies are now ongoing for the design of a R134a recuperation plant. Indeed, R134a represents a major contribution to the gas consumption for particle detection at CERN. During Run 2 a first test was performed with a prototype system (Figure 1) on a real LHC-RPC detector. A R134a recuperation efficiency close to 100% was achieved. Moreover, the recuperated R134a was as pure as the fresh gas. Further tests are needed to investigate the filtering capacity with respect to RPC specific impurities. The following step will be the design and construction of a module allowing storage and re-use of the recuperated gas. In any case, this prototype has paved the way for the design of a final recuperation system for Run 3.    

Figure 1: First prototype of the R134a recuperation plant successfully tested recently on the RPC detector system.

A third research line is based on the long-term replacement of currently used gases for the operation of gaseous detectors. An intense R&D activity is already ongoing since many years in the detector community for finding more environmentally friendly replacements in particular for R134a, SF6 and CF4. Hydrofluoroolefins (HFOs) compounds have been developed by industry to replace R134a as refrigerant fluid and, therefore, they are the first candidates for replacing R134a in our application. However, finding a suitable replacement for the RPC systems at the LHC experiments is particularly challenging because most of the infrastructure (i.e. high voltage systems, cables, front-end electronics) as well as the detectors themselves cannot be easily replaced. Therefore, the R&D is focused on identifying a new mixture able to reproduce the same RPC performance observed with the current R134a based mixture. Encouraging results have been obtained with a partial substitution of the R134a with HFO-1234ze and the addition of a neutral gas (for example CO2). However, a final satisfactory solution is still far from being achieved.

For future RPC applications where, high voltage systems and detectors can be designed without specific constraints, preliminary results obtained with only HFO based mixture are also promising.

In parallel, RPC detector operation with gas recirculation system and new environmentally friendly gases is also under study at the CERN Gamma Irradiation Facility (GIF++) in presence of background radiation similar to the one expected during operation at the LHC experiments.

For cases where gases cannot be recuperated/re-used, systems for their disposal have been developed by industry. They are adopted when such gases after being used in the industrial process are polluted to a level for which the recuperation for re-use is not possible. Unfortunately, many of these gases are very stable compounds and therefore very difficult to dispose. Most important, abatement systems are only solving part of the problem, i.e. the gas emission. Problems like gas availability and price for detector operation are not addressed by abatement systems and these might become the challenge in the coming years due to the phase down policy for such gases.

The different strategies described above should be combined to achieve the highest possible exploitation of the gas, reducing the consumption as well as operational costs and minimizing potential problems due to availability. The challenge in using new environmentally friendly gases is coming from the fact that they behave differently with respect to R134a and therefore current detectors and front-end electronics are not optimized for their use.

Figure 2 shows how the reduction of environmentally unfriendly gas emission evolved in the recent few years as a function of the upgrades performed on the LHC gas systems (excluding the optimization already achieved during the early design phase). During LS2 a further big improvement could come from a reduction of the leak rate at detector level combined with the implementation of R134a recuperation plants.

 

 

Figure 2: 

Reduction of environmentally-unfriendly gas emission in the recent few years as a function of the upgrades performed on different LHC gas systems.

 

Further Reading

[1] "Regulation (EU) No 517/2014 of the European Parliament and of the Council on fluorinated greenhouse gases and repealing Regulation (EC) No 842/2006".

[2] R. Guida and B. Mandelli, "R&D for the optimization of the use of greenhouse gases in the LHC particle detection systems," in 15th Vienna Conference on Instrumentation, Vienna, 2019.

[3] R. Guida, B. Mandelli and G. Rigoletti, "Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background," in 15th Vienna Conference on Instrumentation, Vienna, 2019.