Helium gas mixtures for ring imaging Cherenkov detectors with CsI photocathodes

doi:10.1016/0168-9002(93)90793-H

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 334, Issues 2–3, 1 October 1993, Pages 328-338

https://doi.org/10.1016/0168-9002(93)90793-H Get rights and content

Abstract

Use of He gas mixtures at atmospheric pressure (instead of low-pressure hydrocarbon gases) will ease the construction and operation of large-scale parallel-plate RICH detectors with CsI photocathodes. Due to the low density of He gas the number of primary electrons created by minimum-ionizing particles is very small even at atmospheric pressure. Therefore the Cherenkov detector will be nearly blind to ionizing tracks. However, He alone is not a good counting gas; a suitable quenching component must be added for stable operation of the chamber at high gas gain. Five different quenching gases, CH₄, C₂H₆, isobutane, CF₄ and CO₂ have been tested at a mixing ratio of 5%. The first three gas mixtures show very promising performance. The parallel-plate avalanche chamber (PPAC) can be operated up to gains ≈ 5×10⁵ without secondary avalanches. When the He gas mixture includes 5% of the quenching gas (38 Torr) the efficiency for minimum-ionizing particles is calculated to be < 20% using a Monte Carlo simulation based on measured gas-gain spectra. Experimental investigation shows that the ratio of the quantum efficiency of a CsI photocathode in an atmospheric-pressure He/i-C₄H₁₀ (95/5) gas mixture to that in low-pressure i-C₄H₁₀ gas is ≈ 75% for PPAC operation. The effect of mechanical tolerance on uniformity of gas gain at various pressures is also discussed in detail, and He/i-C₄H₁₀ (95/5) has the least variation of gain with gap thickness of the mixtures tested.

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Citation Excerpt :
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