Small scale time projection chamber setup to test the purity of liquid krypton from the NA62 experiment at CERN

A high liquid krypton purity level is essential for the NA62 calorimeter at CERN to ensure long lifetimes of the ionization electrons in the detector. A small time projection chamber (TPC) system was developed to enable the measurement of liquid krypton purity at the oxygen equivalent ppb level. This paper describes the dedicated cryogenic setup and the procedures required to operate the purity monitor system, while guaranteeing the condensation of krypton at 119 K without freezing it at 115.8 K on the cryocooler-based heat exchanger. The focus is on the materials used, and the instrumentation and pressure safety equipment that allowed the bake-out and purging procedures to reach ppb levels of impurities in the two litre vessel containing the TPC. The setup was integrated in the NA62 experimental area and successfully tested with purified commercially obtained krypton.