University of Cambridge > > Cavendish HEP Seminars > Ultrasonic gas mixture analysis for Cherenkov Refractometry and coolant leak hunting in silicon trackers.

Ultrasonic gas mixture analysis for Cherenkov Refractometry and coolant leak hunting in silicon trackers.

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If you have a question about this talk, please contact Nick Barlow.

In a binary gas mixture at known temperature and pressure the sound velocity is a unique function of the molar concentrations of the two components.The technique is particularly sensitive to variations in  trace concentraton of a heavy vapour additive in a light carrier. Sound pulse transmission in opposite  directions relative to the gas flow allows simultaneous flowmetry and mixture analysis  in the same instrument.

Since the 1980’s Ring Imaging Cherenkov (RICH) detectors have used fluorocarbon gas radiators, due to their high UV transparency and tunable range of refractive index near atmospheric pressure. In the SLD barrel CRID at the SLAC linear collider C5F12 (perfluoropentane: mw 288) was mixed with 20% nitrogen. Continuous  real-time sound velocity monitors (“sonars”) at different heights in the radiator vessel monitored stratification  during fill and recovery of the fluorocarbon component, without the need for gas extraction to a refractometer. During long-term running the sonars allowed correction of the mixture to stabilize the B=1 Cherenkov angle to better  than +/- 0.2 mrad rms. The technique has since been used – less extensively – in other RICH detectors.

The ATLAS silicon tracker uses C3F8 and CO2 evaporative cooling. Leaks of both these fluids  into the N2-purged envelopes of the tracker are detectable by sonar instruments: C3F8 leak  concentrations less than 5.10-5 being routinely detectable in the pixel and SCT envelopes. 

The compressor-driven C3F8 evaporative system is being replaced by a thermosiphon recirculator.  Two sonars are installed in the system: one monitors air ingress into the condenser while the other will montior the combined 0.4 m3/s^-1 C3F8 return flow from the tracker. 

All the sonar instruments have custom electronics cummunicating via Modbus TCP /IP with a Dell  Poweredge R610 SCADA computer running WINCC . The sonar system is being integrated into the ATLAS detector control system and alarm handler. 

We have also demonstrated that the barrel SCT can operate 9C colder with a blend of 75%C3F8/25% C2F6 , giving better tolerance to radiation damage under increased LHC luminosity. Sonar instruments monitor C2F6 /C3F8 blend stability to 0.3%. The sonar flowmeter in the thermosiphon would perform this analysis should C2F6 /C3F8 blends be used in the future.

These instruments are well adapted to many other applications where simultaneous flowmetery and analysis are desired, including hydrcarbon/air mixtures, gas mixtures for MOCVD semiconductor manufacture, diving and clinical anaesthesia.

This talk is part of the Cavendish HEP Seminars series.

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