The Tracker is a subdetector that is located closest to the collision point of the LHC’s particle beams inside the CMS detector. It must face an onslaught of billions of particles flying through it each second of operation. To mitigate the effects of damage from radiation at the higher beam intensities of the LHC from 2015 onwards, the Tracker must be operated at temperatures colder than ever before. This posed a monumental challenge to the Tracker community. But after two years of planning, one year of work and two million Swiss Francs invested, they succeeded and the Tracker is now in shape to operate until 2025!
The CMS Tracker cannot be operated at room temperatures since damage induced by traversing particles would render it inoperable after only a fraction of its foreseen lifetime: electrical currents through the sensors increase linearly with radiation damage. Fortunately, these currents are also exponentially dependent on the temperature, and can be largely reduced by running at low temperatures. During the LHC’s first run from 2010 to early 2013, the Tracker operated at a temperature of +4 °C using its fluorocarbon cooling system.
Several separate projects needed to be completed in parallel to achieve still lower temperatures. The cooling plant located at our experimental site in Cessy was refurbished. New methods for vapour-sealing and insulation were engineered to suppress humidity inside the Tracker volume.
Dry gas (air or nitrogen) is needed to keep humidity away from the delicate electronics. To this end, a new dry-gas plant has been installed by the CMS Technical Coordination team to augment the amount of dry gas supplied to the Tracker volume. This installation provides eight times as much dry gas as the previous system did, and will also allow fine-grained regulation of the flow.
Several hundred high-precision sensors were installed to better monitor the humidity and temperature conditions during cold operation. In addition, all cooling bundles outside the Tracker itself were equipped with heater wires and temperature sensors to guarantee safe operation above the cavern dew point in the future. The fluorocarbon cooling system was completely overhauled.
In late January and early February this year, the Tracker successfully passed the important ‘Master Cold Test’ milestone, running the Strip Tracker at temperatures down to −20 °C with the Pixel Tracker lines going all the way to −25 °C.
The subdetector was monitored continuously during this exercise and performed as expected. It was also demonstrated that cooling the Tracker did not affect the temperature of the surrounding layers of the Electromagnetic Calorimeter, which operate at +18 °C.
The environmental conditions allow operation at −25 °C and the detector has been successfully operated at −20 °C. To minimise thermal stress but still not suffering any radiation reverse effects we are discussing operating the Strip Tracker at −15 °C and the Pixel Tracker at −20 °C for the coming years.
With this milestone, the Tracker project has completed the bulk of its work for Long Shutdown 1 of the LHC, resulting in the ability to operate the detector cold with a sufficient safety margin. We are now ready to track particles until Long Shutdown 3!
Submitted by Nicola Bacchetta, Erik Butz, Francesco Palmonari, Antti Onnela and Frank Hartmann