The ATLAS IBL CO₂ cooling system

The ATLAS Pixel detector has been equipped with an extra pixel layer in the space obtained by a smaller radius beam pipe. This new pixel layer called the Insertable B-Layer (IBL) was installed in 2014 and is operational in the current ATLAS data taking. The IBL detector is cooled with evaporative CO₂ and is the first of its kind in ATLAS. The ATLAS IBL CO₂ cooling system is designed for lower temperature operation (< −35^oC) than the previous developed CO₂ cooling systems in High Energy Physics experiments. The cold temperatures are required to protect the pixel sensors for the expected high radiation dose received at an integrated luminosity of 550 fb¹. This paper describes the design, development, construction and commissioning of the IBL CO₂ cooling system. It describes the challenges overcome and the important lessons learned for the development of future systems which are now under design for the Phase-II upgrade detectors.