The Omega RICH☆

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Abstract

A large-aperture RICH for identification of secondary particles is in operation at the Omega spectrometer since 1984. Photons are detected in drift chambers with quartz windows, using TMAE-loaded counting gases. The RICH was used by two experiments, WA69 and WA82, until 1988. It was then equipped with new drift chambers and mirrors and is in use since 1990 mainly for the hyperon beam experiment WA89. The present setup is described in more detail, and efficiencies, resolutions and particle separation achieved are discussed.

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    Considered a strategic material, it was scarcely available on the market until 1980, when D. F. Anderson investigated it as photo-converter for RICH detectors [22]. Despite these drawbacks, the long experience acquired allowed a safe and reliable handling and utilization of these compounds for about twenty years in most of the UV RICH detectors operated in particle physics experiments at CERN and SLAC [23–25]. The fluorescent light emitted by TMAE, resulting from the de-excitation of excited states created by electron impact collisions during the avalanche amplification process in the MWPC, was exploited for RICH counters with optical readout [26].

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    TMAE has a low photo-ionization threshold (5.3 eV) and is compatible with the use of fused silica windows: the resulting effective wavelength range for photon detection is between 160 nm and 220 nm. The largest RICH detectors ever built: the DELPHI RICHs [10], the SLD CRID [11] and the OMEGA RICH [12,13] used TMAE and a PD based on the TPC RICH concept [14]. Due to the low TMAE vapor pressure at room temperature the photon conversion gap needs to be centimeters thick and the detectors were operated at high temperature (40 degrees or even 50 degrees in the case of CERES RICH [15]).

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    TMAE instead has a low photo-ionization threshold (5.3 eV), making it compatible with the use of fused silica windows, thus providing an effective wavelength range for photon detection from 160 to 220 nm. The largest RICH detectors ever built: DELPHI RICH [11] and SLD CRID [12] as well as the OMEGA RICH [13,14] used TMAE and a PD based on the TPC RICH concept [15]. TMAE also emits visible light: TMAE-based optical readout of RICH detectors [16] was implemented in the NA35 Experiment [17].

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    Conclusions and perspectives in the field are given in Section 7. The photoconverting vapours used in gaseous photon detectors for Cherenkov imaging counters are TMAE (Tetrakis Dimethylamine Ethylene), for example employed in OMEGA RICH [1], DELPHI RICHes [2] and SLD CRID [3], and TEA (Tri-Ethyl-Amine), used in E605 RICH [4] and in CLEO-III RICH [5]. The choice of gaseous detectors allowed to instrument large surfaces, more than 30 m2 in DELPHI RICHes and SLD CRID.

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    Second generation detectors, based on the (positive and negative) experiences gained earlier, were used at the end of the 1980s and beginning of 1990s. Examples include an upgraded Omega RICH [18] used by WA89 [20] and WA94 [21], DELPHI [22], SLD-CRID [23,24], CERES [25,26], and SPHINX [27]. These detectors reached about half of the originally expected number of detected photons, but all the losses could be eventually accounted for by carefully determining all the operational parameters; one large problem was the TMAE quantum efficiency and electron attachment length depending on the cleaning procedure of the TMAE [28,29].

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☆

Work supported by the Bundesministerium für Forschung und Technologie (codes 05 5HD15I, 06 HD524I, 06 MZ5265 TP2).

∗∗

Work supported by the Bundesministerium für Froschung und Technologie (codes 05 5HD151, 06 HD524I, 06 MZ5265 TP2).

1

Now at Fermi Nat. Acc. Lab., Batavia, IL., USA.

2

On leave from Lebedev Inst., Moscow, Russia.

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