Eur. Phys. J. Plus (2023) 138: 1141
https://doi.org/10.1140/epjp/s13360-023-04703-w

Regular Article

The “green” use of fluorocarbons in Cherenkov detectors and silicon tracker cooling systems: challenges and opportunities in an unfolding era of alternatives

Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France

^a gregh@cppm.in2p3.fr

Received: 30 April 2023
Accepted: 16 November 2023
Published online: 19 December 2023

Abstract

Saturated fluorocarbons (SFCs) of form C_nF_(2n+2) are chosen for their optical properties as Cherenkov radiators, with C₄F₁₀ and CF₄ currently used at CERN in the COMPASS and LHCb ring imaging Cherenkov detectors. Their non-conductivity, non-flammability and radiation-resistance also make SFCs ideal coolants: C₆F₁₄ liquid cooling is used in all LHC experiments, while C₃F₈ is used for the evaporative cooling of TOTEM and the ATLAS silicon tracker. These fluids, however, have high global warming potentials (5000–10000*GWP_CO2), and represented around 36% of CERN’s CO₂-equivalent emissions in 2018. There is thus an impetus to reduce their use, losses in purification and wastage through leaks, through improved monitoring and closed circulation system design. Newer spur-oxygenated fluoro-ketones, for example from the 3 M NOVEC^® range, with C_nF_2nO structures, can offer similar performance to SFCs with but with very low, or zero GWP. Although these fluids do not yet exist in large quantities over the full C_nF₂ “matrix” the radiation tolerance and thermal performance of NOVEC 649 (C₆F₁₂O) was sufficiently promising for it to be chosen as a C₆F₁₄ replacement for cooling silicon photomultipliers. Additionally, subject to optical testing, NOVEC 5110 (C₅F₁₀O) could (if blended with nitrogen) replace both C₄F₁₀ and CF₄ in Cherenkov detectors. Lighter molecules (for example C₂F₄O, with similar thermodynamics to C₂F₆)—if and when available in industrial quantities—might allow lower temperature operation than evaporative CO₂ in future silicon trackers operated at very high luminosity. Ultrasonic gas mixture analysis is very sensitive to concentration changes of a heavy vapour in a light carrier, and is used—in the only such fluorocarbon coolant leak monitoring system operating at LHC—for real-time monitoring of C₃F₈ coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes. A typical C₃F₈ sensitivity of better than 10⁻⁵ is achieved. Advanced new ultrasonic algorithms allow measurement of the concentrations of a pair of gases of particular interest on top of a varying known baseline of other gases. The technique is thus of considerable value in leak monitoring and could be used to blend fluoro-ketones with nitrogen or argon to reduce the GWP “load” of large volume atmospheric pressure gas Cherenkov radiators without the recourse to higher-pressure noble gas approaches. This paper outlines an approach to GWP reduction with fluoro-ketone fluids and the blending of heritage SFCs or fluoro-ketones with lighter gases using ultrasonic monitoring and control. Possible avenues for the use of fluoro-ketones in liquid phase and evaporative cooling of silicon trackers are discussed.