Abstract
COMPASS RICH-1 is a large size gaseous Imaging Cherenkov Detector providing hadron identification in the range from 3 to 55 GeV/c, in the wide acceptance spectrometer of the COMPASS Experiment at CERN SPS.It uses a 3 m long C4F10 radiator, a 21 m2 large VUVmirror surface and two kinds of photon detectors: MAPMTsand MWPCs with CsI photocathodes, covering a total of 5.5 m2.It is in operation since 2002 and its performance increased thanks to progressiveoptimization and to a major upgrade of its photon detection system, implementedin 2006; a new upgrade is foreseen for 2016, with the use of MPGD-basedphoton detectors.The main characteristics of COMPASS RICH-1 components are described andthe most critical aspectsrelated to the C4F10 radiator gas system, to the mirrors and their alignment,as well as the performance of the photon detectors are presented and discussed.The response of the MWPCs and the observed evolution of the effective quantumefficiency of the CsI photocathodes is analyzed.The properties and performance of the MAPMTs with individual fused lens telescopesare presented together with the readout characteristics.The PID performance of COMPASS RICH-1 is discussed and the future upgrade programis mentioned.
Highlights
The RICH-1 vessel and the gas radiatorHadron identification in the high momentum domain imposes the use of a gas radiator; the choice of MWPCs with CsI photocathodes (the only affordable technology for covering large photosensitive surfaces at that time), coupled to the specific COMPASS spectrometer characteristics (in particular the large acceptance) lead to the need of a large size radiator volume (about 80 m3)
It provides π-K separation from 3 to 55 GeV/c over an angular acceptance of ±250 mrad in the horizontal plane and ±180 mrad in the vertical plane, with beam rates up to 40 MHz and trigger rates up to 20 kHz
(PDs), based on the use of MWPCs and CsI, are introduced, their performance is discussed and the results from the analysis of long term CsI QE variations are presented in detail; section 5 is dedicated to the Photon Detectors (PDs) covering the central region of RICH-1, based on the use of MAPMTs and to a short presentation of the RICH-1 readout and DAQ systems
Summary
Hadron identification in the high momentum domain imposes the use of a gas radiator; the choice of MWPCs with CsI photocathodes (the only affordable technology for covering large photosensitive surfaces at that time), coupled to the specific COMPASS spectrometer characteristics (in particular the large acceptance) lead to the need of a large size radiator volume (about 80 m3). Light transmission values larger than 88% in the VUV wavelength domain of the CsI-based photon detectors (165–210 nm), close to the limit provided by Rayleigh scattering process, are routinely obtained: a typical example of a radiator transparency measurement result can be seen in figure 7 where the transparency of 187 cm of the gas as function of the wavelength is checked and fitted to extract the estimated contamination levels for the UV absorbing impurities. During the entire running period regular checks and immediate emergency interventions are granted by COMPASS RICH-1 experts on call
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