Evaporative CO2 cooling system for the upgrade of the CMS pixel detector at CERN

Carbon dioxide (CO₂) as evaporative coolant has gained interest as a technique for cooling of high-energy particle physics detectors and front-end electronics. Silicon tracking detectors need to be maintained at sub-zero temperature to enhance their lifetime in the presence of radiation damage. In addition, the material budget allocated to infrastructure must be as small as possible, to allow maximum transparency for tracking the trajectories of particles. Evaporative cooling is clearly the best method to meet these goals, and CO₂ as coolant is an excellent option for this application as it can withstand a large amount of radiation and has excellent thermal behaviour in small diameter tubes. CO₂ evaporative cooling has been selected for the next generation CMS Pixel detector, due in ~2016 to replace the present detector which is cooled with liquid C₆F₁₄. The design requirements for the new detector are an operating temperature of -10 °C on the silicon pixel sensor (-20 °C on the coolant) and a total power of about 15 kW. Following the successful applications in AMS and LHCb Velo projects, the 2-Phase Accumulator Controlled Loop method (2PACL) has been chosen and scaled up by a factor 10 in cooling power. Liquid CO₂ is pumped from the cooling plant to the detector along 40-meter transfer lines. At the evaporators inside the detector, low vapour quality CO₂ is fed at a constant pressure, with no need of active components in the vicinity of transfer lines the particle interaction region. This paper describes the general design of the Pixel system and the on-going tests of the detector on-board evaporators and long transfer line prototypes. This development is part of the CMS Pixel Upgrade project, and it is being carried out in the framework of the CMS Pixel Collaboration.