M. Capeans

CONSTRUCTION, TEST AND COMMISSIONING OF THE TRIPLE-GEM TRACKING DETECTOR FOR COMPASS

The Small Area Tracking system of the COMPASS experiment at CERN includes a set of 20 large area, fast position-sensitive Gas Electron Multiplier detectors, designed to reliably operate in the harsh radiation environment of the experiment. We describe in detail the design, choice of materials, assembly procedures and quality controls used to manufacture the devices. The test procedure in the laboratory, the performance in test beams and in the initial commissioning phase in the experiment are presented and discussed.

The gas electron multiplier (GEM)

We describe operating principles and results obtained with a new detector element: the Gas Electron Multiplier (GEM). Consisting of a thin composite sheet with two metal layers separated by a thin insulator, and pierced by a regular matrix of open channels, the GEM electrode, inserted on the path of electrons in a gas detector, allows the transfer of charge with an amplification factor approaching ten. Uniform response and high rate capability are demonstrated. Coupled to another device, multiwire or micro-strip chamber, the GEM electrode permits higher gains or less critical operation; separation of the sensitive (conversion) volume and the detection volume have other advantages: a built-in delay (useful for triggering purposes), and the possibility of applying high fields on the photo-cathode of ring imaging detectors to improve efficiency. Multiple GEM grids in the same gas volume allow large amplification factors to be achieved in a succession of steps, leading to the realization of an effective gas-filled photomultiplier.

Development of the gas electron multiplier (GEM)

We describe recent developments of the gas electron multiplier (GEM), a thin composite mesh acting as proportional avalanche amplifier in gas counters. In beam tests we have verified the excellent efficiency, time resolution and localization accuracy for a GEM with micro-strip read-out. Efficiency, localization accuracy and operation in strong magnetic fields has been verified; operation at rates above 10/sup 6/ Hz/mm/sup 2/ and lifetimes corresponding to at least 10 mC/cm of collected charge have been demonstrated. Refinements in the manufacturing technology have permitted the realization of large size detectors (27 by 25 cm/sup 2/), to be used in conjunction with microstrip gas chambers. With an improved design, stable gains above two thousand have been reached (GEM2000); larger gains can be obtained increasing the thickness of the foils, cascading two GEMs at some distance or in electrical contact. Further developments of the technology and prospective applications are discussed.

Ageing of microstrip gas chambers: problems and solutions

Abstract The experimental setup and the procedures used for studying the long-term behaviour of micro-strip gas chambers under sustained irradiation are described in detail. The most significant measurements on ageing obtained in a variety of conditions are reported, and a tentative interpretation of the results is presented. The relevance of these findings for the conception, construction and use of MSGCs trackers in high luminosity LHC detectors is discussed.

High rate operation of micro-strip gas chambers on diamond-coated glass

Abstract Very high rate operation of micro-strip gas chambers can be achieved using slightly conducting substrates. We describe preliminary measurements realized with detectors manufactured on boro-silicate glass coated, before the photo-lithographic processing, with a diamond layer having a surface resistivity of around 10 14 Ω/. Stable medium-term operation, and a rate capability largely exceeding the one obtained with identical plates manufactured on uncoated glass are demonstrated. If these results are confirmed by long-term measurements, the diamond coating technology appears very attractive since it allows, with a moderate cost overhead, to use thin, commercially available glass with the required surface quality for the large-scale production of gas micro-strip detectors.

Development of micro-strip gas chambers for high rate operation

Abstract We describe the developments of micro-strip gas chambers able to withstand the very high rates foreseen for operation as vertex detector in high luminosity experiments, and for applications in medical diagnostics. To avoid surface charging-up processes, we have used as supports electron-conducting glass with resistivity in the range 109 to 1012 Ω cm, and boro-silicate glass with thin coatings of lead silicate with surface resistivity between 1014 and 1016 Ω/□. A systematic research has been undertaken to find the purity levels of the gas filling and of the manufacturing materials necessary for long-term stability of operation, the goal being 10 years of operation at LHC (or about 140 mC cm−1 of collected charge). In particular, we have tested high-grade polymers that can be injection-moulded into the shape required to make MSGC frames, and epoxies that satisfy the stringent outgassing requirements. A strong dependence of ageing from the charge rate used in the irradiation has been found, indicating that measurements realized at high current densities may be too optimistic in terms of expected lifetime of the detectors; this seems to be particularly true for MSGCs manufactured on high resistivity boro-silicate glass.

On some factors affecting discharge conditions in micro-strip gas chambers

Abstract We describe the results of an investigation on the influence of geometricla parameters on the maximum gain that can be attained in a micro-strip gas chamber. Single electron emission from the cathode was confirmed to be a precursor of discharge, particularly for gas mixtures with a high fraction of quencher, confirming our previous observations. In contradiction however with the results of a theoretical model proposed in earlier work, we found that the maximum gain is obtained in a narrow range of cathode strip width. This suggests the presence of other mechanisms initiating a discharge, such as photon- or ion-initiated feedback. The influence of strongly ionising irradiation on discharge conditions has also been investigated; we have observed an increasing probability of discharge in detectors irradiated with α particles, releasing around 1 MeV in the gas, as compared to 6 keV X-rays. This results in a reduction by a factor of 2 of the maximum gain at which a MSGC can be safely operated, a concern for the use of micro-strip chambers in vertex detectors for colliders where low energy, heavily ionizing particles and thermal neutrons are produced together with minimum ionizing particles.

Effects of outgassing from some materials on gas chamber ageing

Abstract Several different materials that might be used for assembling micro-strip gas chambers (MSGCs) for high luminosity experiments were tested by measuring the gain of an irradiated wire counter when each sample was put into contact with the gas upstream of the detector. The results show a fast effect on the gain of the counter for some of the materials tested, whereas others, like Stesalit, Vectra, and the epoxy Epotek 505, do not seem to emit dangerous vapours, even at high temperatures.

Results of wire chamber ageing tests with CH4− and DME-based gas mixtures

Abstract Results are presented of ageing tests performed on single-wire proportional counters under controlled conditions. The rate of the detector ageing with methane mixtures has been found to be independent of the anode and cathode materials used, and also of the purity of the gas. The rate of ageing for DME mixtures, on the other hand, appears to depend on the amount of gas used: it is small when the DME bottle is full, but increases as the cylinder empties. Addition of some water vapour to the Ar-DME mixture provided good lifetime, independently of the amount in the bottle. An explanation of this observation, based on assumptions on the fractional distillation of impurities, is provided.

High rate operation and lifetime studies with micro-strip gas chambers

Abstract Micro-Strip Gas Chambers manufactured on supports with low resistivity, 10 9 to 10 12 Ω cm, can withstand without gain loss due to charging-up the very high rates expected in tracking detectors at CERNs Large Hadron Collider. Several technologies allowing to realize supports in this range of resistivity are described, namely electron-conducting glass and thin-layer conductive coatings. At very high radiation fluxes, the formation and deposit on electrodes of thin insulating layers cause premature deterioration of the detector. The ageing rate is found to be extremely sensitive to the purity and nature of the filling gas, and can be kept small only by a careful selection of the manufacturing materials. Experimental results on ageing are presented for several types of detectors.