Saturated fluorocarbons (SFCs: of chemical form C_nF_(2n+2)) are chosen for their optical properties (refractive index and UV transparency) for use as Cherenkov radiators, with C4F10 and CF4 used in the COMPASS and LHCb RICH1&2 ring imaging Cherenkov detectors to provide mu/pi/K/p particle identification over a wide momentum range.

Non-conductivity, non-flammability and radiation resistance also make SFCs ideal coolants, with C6F14 liquid used in all LHC experiments, while C3F8 evaporatively cools the ATLAS silicon tracker. 

These fluids however have high GWPs (>5000*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, via improved monitoring and closed circulation system design.

While not yet industrialised over the full C_nF_2nO range, spur-oxygenated  fluoro-ketones can offer similar performance at very low, or zero GWP. The radiation tolerance and thermal performance of 3M 'NOVEC 649' (C6F12O) was sufficiently promising for it to be chosen by CERN to replace C6F14. Subject to optical testing, 3M 'NOVEC 5110'  (C5F10O) - blended with nitrogen and monitored in real time by sound velocity gas analysis - might replace C4F10 and CF4 in RICH detectors.

Ultrasonic gas mixture analysis is very sensitive to concentration changes of a heavy vapour in a light carrier, and is used  for real-time monitoring of C3F8 coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes, where 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 could be used to blend fluoro-ketones with nitrogen or argon to reduce the GWP “load” of large volume Cherenkov radiators. The technique is also of interest in xenon-based anaesthesia, whch has a similar problematic.

Light fluoro-ketone  molecules (e.g. C2F4O, with similar thermodynamics to C2F6) - might allow lower temperature, 0GWP operation than evaporative CO2 in Si trackers operated at high luminosity.

This  seminar 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.