B. Ketzer

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.

Discharge studies and prevention in the gas electron multiplier (GEM)

The gas electron multiplier (GEM) used as single proportional counter or in a cascade of two or more elements, permits to attain high gains and to perform detection and localization of ionizing tracks at very high radiation rates. As in other micro-pattern detectors, however, the occasional occurrence of heavily ionizing trails may trigger a local breakdown, with possible harmful consequences on the device itself and on the readout electronics. This paper describes a systematic investigation of the discharge mechanisms in single and multiple GEM structures, and suggests various strategies to reduce both the energy and the probability of the discharges.

High rate X-ray imaging using multi-GEM detectors with a novel readout design

Abstract Modern micropattern gas counters are able to detect soft X-rays with high spatial resolution. Based on the principle of gas amplification, this detector type has a wide dynamic range and provides energy selection during image reconstruction. One of the most recent developments is the Multi-GEM, an efficient, high-rate capable micropattern gas detector. Owing to the separation of gas amplification and readout stage, this detector allows high flexibility in the geometry of the readout structure. With a two-dimensional projective readout, GEM detectors have shown good capability in providing X-ray absorption radiographies of small mammals. We present a detector with a novel design where the readout board consists of micropads connected to three layers of strips in such a way that two-dimensional spatial reconstruction and an unambiguous resolution of events with multiple hits is possible.

Performance of GEM detectors in high intensity particle beams

Abstract We describe extensive tests of Double Gas Electron Multiplier (GEM) and Triple GEM detectors, including large size prototypes for the COMPASS experiment, exposed to high intensity muon, proton and pion beams at the Paul Scherrer Institute and at CERN. The measurements aim at detecting problems possibly appearing under these harsh operating conditions, the main concern being the occurrence of discharges induced by beam particles. Results for the dependence of the probability for induced discharges on the experimental environment are presented and discussed. Implications for the application of GEM detectors in experiments at high luminosity colliders are illustrated.

Triple GEM tracking detectors for COMPASS

The small-area tracker of COMPASS, a high-luminosity fixed target experiment at CERNs SPS, includes a set of 20 large-size (31 /spl times/ 31 cm/sup 2/) gas electron multiplier (GEM) detectors. Based on gas amplification in three cascaded GEM foils, these devices permit to obtain high gain and good spatial resolution even at very high particle fluxes. A two-coordinate projective readout yields, for each track, highly correlated signal amplitudes on both projections, allowing to resolve multiple hits in high occupancy regions close to the central deactivated area of 5 cm diameter. At the same time, the material exposed to the beam is minimized. Splitting the amplification in three cascaded stages permits to achieve a gain of /spl sim/ 8000, necessary for efficient (> 98%) detection of minimum ionizing particles on both coordinates, already at relatively moderate voltages across individual GEM foils. As a consequence, the probability of a gas discharge to occur when a heavily ionizing particle enters the detector volume, is reduced by more than one order of magnitude at a given gain compared to the initially foreseen double GEM structure. In conjunction with other strategies resulting from extensive R&D on discharge phenomena, we were able to further reduce both the energy and the probability of such breakdowns. In order to completely exclude permanent damage to the front-end chip by the rare event of a discharge fully propagating to the readout strips, an external electronic protection circuit is used. The operational characteristics of these detectors were examined both in the laboratory and in the beam, where a spatial resolution for minimum ionizing particles of (46 /spl plusmn/ 3) /spl mu/m and a time resolution of /spl sim/ 15 ns were achieved. For the 2001 run of COMPASS, a total of 14 triple GEM detectors have been installed. First results from the commissioning phase in the high-intensity /spl mu/ beam are presented.

DEVELOPMENT AND APPLICATIONS OF THE GAS ELECTRON MULTIPLIER

The Gas Electron Multiplier (GEM) has been recently developed to cope with the severe requirements of high luminosity particle physics experimentation. With excellent position accuracy and very high rate capability, GEM devices are robust and easy to manufacture. The possibility of cascading two or more multipliers permits to achieve larger gains and more stable operation. We discuss major performances of the new detectors, particularly in view of possible use for high rate portal imaging and medical diagnostics.

GEM detectors for COMPASS

For the small-area tracking of particles within COMPASS (common muon and proton apparatus for structure and spectroscopy), a new fixed target experiment at CERN/SPS, several large-size(31/spl times/31 cm/sup 2/) detectors based on the gas electron multiplier (GEM), have been built. These new devices, consisting of several GEM amplification stages with a two-coordinate readout, combine good spatial resolution with high rate capability, which is required by the large particle flux near the beam. At the same time, the material exposed to the beam is minimized in order not to spoil the mass resolution of the spectrometer. The first detectors out of a total of 20 were subject to extensive tests in the beam and in the laboratory, showing that full (i.e., >98%) detection efficiency for minimum ionizing particles can be achieved at a total effective gain of 6000. Ongoing research work focuses on discharges triggered by heavily ionizing particles entering the detectors. Systematic studies of the energy released in discharges and their probability of occuring at all as a function of a variety of parameters suggest several means to minimize their impact on detector performance. First results of the operational characteristics of these detectors in the real COMPASS beam are presented.

Micropattern gaseous detectors in the COMPASS tracker

The tracking of particles in the region close to the high-intensity beam of the COMPASS experiment at CERN is based on two novel types of micropattern gaseous detectors, the Micromegas and the GEM. Chosen for their high localization accuracy and rate capability, intrinsic to this technology of highly granular gaseous devices, their large active area of up to 40 � 40 cm 2 and small material budget offer additional advantages for tracking of particles in a high-luminosity experiment. The basic principles of these detectors as well as the design adopted for the COMPASS experiment, aimingat optimization of operation accordingto their positions in the spectrometer, are presented. Means to minimize the probability of gas discharges, and to reduce their impact on detector operation, as implemented for both detector types, are discussed. For the 2001 run of COMPASS, over 50% of the total number of detectors required for the full setup was installed and successfully operated. First results concerningthe operational characteristics in the COMPASS muon beam are presented. r 2002 Elsevier Science B.V. All rights reserved.

A fast Tracker for COMPASS based on the GEM

Abstract The small area tracking system of COMPASS, a new high-luminosity fixed target experiment at CERNs SPS accelerator, comprises 20 large-size triple GEM (Gas Electron Multiplier) detectors. It was completed and fully operational for the first time for the 2002 muon run. We report on the performance of these chambers in the high-intensity beam and give first results on efficiencies as well as spatial and time resolutions measured for the full set of detectors under various beam conditions.

Optimisation of the gas electron multiplier for high rate applications

Abstract The construction and performance of large size GEM detectors for the COMPASS experiment is described. Based on the experience gained during the operation of these detectors in high rate muon, proton, and pion beams we discuss the suitability of their use in harsh radiation environments.