“Green” use of fluorocarbons in Cherenkov detectors and silicon tracker cooling systems: challenges and opportunities.

Saturated fluorocarbons (SFCs: chemical form CnF(2n+2)) are chosen for their optical properties as Cherenkov radiators, with C4F10 and CF4 used in the COMPASS and LHCb Ring Imaging Cherenkov detectors at CERN. Their non-conductivity, non-flammability and radiation resistance also make them ideal coolants, with C6F14 liquid used in all LHC experiments, and C3F8 used as an evaporative coolant in the ATLAS silicon tracker. While SFCs have high Global Warming Potentials (GWP typically > 5000*CO2), fluoro-ketones (FKs: of chemical form CnF2nO) can offer similar performance at very low, or zero-GWP. This paper considers use of heavy SFC and FK vapours in Cherenkov detectors. The blending of high-order (C > 4) SFC or FK vapours with a light carrier gas is explored to replicate the refractivities of CF4 and C4F10, to reduce or eliminate the GWP “load” (in equivalent tonnes of CO2) in large Cherenkov radiator volumes. Subject to optical testing, 3M NOVEC® 5110 (C5F10O) - blended with N2 and controlled in real time using sound velocity gas mixture analysis - could replace C4F10 and CF4 in RICH detectors. New, non-cyclic isomers of C4F8O could directly replace C4F10 and - blended with N2 - also replace CF4. Noting the impending EU restrictions on fluorinated compounds, and (2025) withdrawal of 3M Corp. from the PFAS (Per- and poly-fluoroalkyl substances) market, radiator GWP load reduction through use of legacy stocks of C5F12 & C4F10 - blended with N2 to replicate the respective refractivities of C4F10 & CF4 - is also considered. The radiation tolerance and thermal performance of 3M NOVEC® 649 (C6F12O) liquid was sufficiently promising for to be considered to replace C6F14 in liquid cooling applications at CERN. Although not industrialized over the full CnF2nO range, lighter (C < 4) fluoro-ketone molecules - for example C2F4O isomers, with similar thermodynamics to C2F6, and subject to toxicity, materials compatibility and low-GWP verifications - might allow lower operating temperatures than possible with evaporative C3F8 or CO2 for the cooling of future silicon trackers operating in high luminosity environments.