Sensors for monitoring water and oxygen in the VUV spectrometer of ALICE-HMPID detector at CERN-LHC

The ALICE HMPID (High Momentum Particle Identification) detector is a RICH (Ring Imaging Cherenkov) counter exploiting perfluorohexane (C₆F₁₄) liquid as Cherenkov radiator to identify charged hadrons in heavy ions and p-p collisions at the CERN-LHC. A dedicated liquid circulation system in conjunction with a dual beam VUV spectrometer, ensure the highest radiator´s Optical Transparency in the wavelength range 160-220 nm, where the RICH detector operates. The spectrometer operates under purge conditions by flushing continuously dry Argon and monitoring carefully water and oxygen levels of the gas, that may cause adverse conditions in transmittance measurements. Special care has been taken in the choice of water and oxygen sensors in order to ensure fast response, sensitivity to ppm contaminations, selectivity and chemical resistance. The purpose of this paper is to show the performance of oxygen and water monitoring system and the sensitivity to low contamination, at ppm levels, for ensuring accurate, stable and reproducible measurements.