The Ring Imaging Cherenkov Detectors for LHCb

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The success of the LHCb experiment depends heavily on particle identification over the momentum range 2 100 GeV/c. To meet this challenge, LHCb uses a Ring Imaging Cherenkov (RICH) system composed of two detectors with three radiators. RICH1 has both aerogel and gas (C4F10) radiators, while RICH2 has only a gas (CF4) radiator. The design of RICH1 is almost complete, whereas RICH2 has been constructed and installed (Nov 2005). Novel Hybrid Photon Detectors (HPDs) have been developed in collaboration with industry to detect the Cherenkov photons. A silicon pixel detector bump-bonded to a readout chip is encapsulated in a vacuum tube. A bi-alkali photocathode is deposited on the inside of the quartz entrance window to convert photons in the range 200 600 nm. The pixel chip is manufactured in 0.25 µm deep-submicron radiation-tolerant technology and consists of 1024 logical pixels, each pixel having an area of 0.5 mm ◊ 0.5 mm. Photo-electrons are accelerated by a 20 kV potential, resulting in a signal of typically 5000 electrons that is amplified and digitized at the LHC speed of 40 MHz. The HPDs are enclosed in iron shielding and Mumetal cylinders surround the individual detectors to protect them from magnetic fields within the external shielding of up to 50 mT. The mass production of a total of 484 HPDs required for the two RICH detectors has already commenced. The design and current status of the LHCb RICH system will be reviewed. Results obtained using prototype HPDs to detect Cherenkov light in particle test beams using the full LHCb readout chain will be presented. Finally, the expected performance of the LHCb RICH system, obtained from realistic simulation, will be shown.