A study of the RICH gas refractive index

Abstract

The Cherenkov RICH detector of the COMPASS experiment uses C 4F 10 as radiator gas. The refractive index of the radiator gas varies with temperature, atmospheric pressure and gas purity. Accurate knowledge of these parameters is essential for the particle identification performance. A modified Jamin interferometer was proposed, constructed and tested to allow on-line refractive index measurement. This interferometer, to be connected to the COMPASS RICH vessel, has a 30 cm active length and is very stable and very vibration-resistant. It uses a He–Ne laser to measure the refractive index at 633 nm with an accuracy of better than 10 −6. The value of the refractive index for all wavelengths of Cherenkov spectra can be calculated from the dispersion curve and the measured value. Because COMPASS RICH is equipped with both PMTs and CsI-based gaseous photon detectors, the average values of the refractive index for the two types of detectors are different: n=1.001345 for PMTs and n=1.001528 for CsI. The Cherenkov angle distribution of the detected photons and the associated chromatic errors for both the PMT and CsI detectors were calculated as a function of the particle momentum.