A Versatile Evaporative Cooling System Designed for Use in an Elementary Particle Detector

An evaporative cooling system developed for operation and qualification testing of silicon pixel and microstrip detectors for the inner tracking detector of the CERN ATLAS spectrometer is described. Silicon detector substrates must be continuously operated between 0 and −7◦C in the high radiation environment near the circulating beams at the CERN Large Hadron Collider (LHC). This requirement imposes unusual constraints on the cooling system and has led to the choice of perfluoron- propane (C3F8) refrigerant, which combines good chemical stability under ionizing radiation with high dielectric strength and nonflammability. Since the silicon detectors must also be of extremely light construction to minimize undesirable physics background, coolant tubes are of thin (200μm) aluminum wall, while evaporative operation allows a very low circulating coolant mass-flow (1–3 g · s−1/100W to evacuate). The assembled detector arrays will undergo qualification tests at room temperature before installation in the ATLAS spectrometer. The cooling system is “dualfuel,” and can also be operated with perfluoro-n-butane (C4F10) refrigerant, offering a reduced evaporation pressure (1.9 bar) compared to that of C3F8 (6.5 bar at 15◦C).