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Article Dans Une Revue The European Physical Journal Plus Année : 2023

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

Résumé

Saturated fluorocarbons (SFCs) of form C$_{n}$F$_{(2n+2)}$ are chosen for their optical properties as Cherenkov radiators, with C$_{4}$F$_{10}$ and CF$_{4}$ 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: C$_{6}$F$_{14}$ liquid cooling is used in all LHC experiments, while C$_{3}$F$_{8}$ 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 CO$_{2}$-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$_{n}$F$_{2n}$O 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$_{n}$F$_{2}$ “matrix” the radiation tolerance and thermal performance of NOVEC 649 (C$_{6}$F$_{12}$O) was sufficiently promising for it to be chosen as a C$_{6}$F$_{14}$ replacement for cooling silicon photomultipliers. Additionally, subject to optical testing, NOVEC 5110 (C$_{5}$F$_{10}$O) could (if blended with nitrogen) replace both C$_{4}$F$_{10}$ and CF$_{4}$ in Cherenkov detectors. Lighter molecules (for example C$_{2}$F$_{4}$O, with similar thermodynamics to C$_{2}$F$_{6}$)—if and when available in industrial quantities—might allow lower temperature operation than evaporative CO$_{2}$ 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 C$_{3}$F$_{8}$ coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes. A typical C$_{3}$F$_{8}$ 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.
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Dates et versions

hal-04411769 , version 1 (31-01-2024)

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G.D Hallewell. The “green” use of fluorocarbons in Cherenkov detectors and silicon tracker cooling systems: challenges and opportunities in an unfolding era of alternatives. The European Physical Journal Plus, 2023, 138 (12), pp.1141. ⟨10.1140/epjp/s13360-023-04703-w⟩. ⟨hal-04411769⟩
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