Speaker
Description
Saturated fluorocarbons (SFCs:C_nF_(2n+2)) are chosen for their optical properties as Cherenkov radiators, with C_4F_10 and CF_4 used in the COMPASS and LHCb RICH detectors. Non-conductivity, non-flammability and radiation resistance also make SFCs ideal coolants: C_6F_14 liquid cooling is used in all LHC experiments, while C3F8 is used for the evaporative cooling in TOTEM and the ATLAS silicon tracker. These fluids, however, have high GWPs (5000–10000*CO_2), and represented around 37% of CERN’s CO_2-equivalent emissions in 2022. There is thus an impetus to reduce their use and losses through improved monitoring and circulation system design.
Spur-oxygenated fluoro-ketones, 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 CnF2n “matrix” the radiation tolerance and thermal performance of C_6F_12O was initially promising enough to be chosen as a C_6F_14 replacement for cooling silicon photomultipliers. Additionally, subject to optical testing, C_5F_10O could (if blended with nitrogen) replace both C_4F_10 and CF_4 in Cherenkov detectors. Lighter molecules (for example C_2F_4O, with similar thermodynamics to C_2F_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_3F_8 coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes. A typical C_3F_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, flammable or expensive noble gas approaches.
An approach is outlined 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.
