A. Bressan

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 behavior and discharge limits in micro-pattern detectors

Abstract We present and discuss a set of systematic measurements, carried out with gaseous proportional micro-pattern detectors, in order to assess their maximum gain when irradiated with high-rate soft X-rays and heavily ionizing alpha particles. The inventory of detectors tested includes: micro-strips, micromegas, micro-dot, gas electron multiplier, CAT (compteur a trous), trench (or groove), micro-CAT (or WELL) detectors, as well as systems with two elements of gaseous amplification in cascade. We confirm the general trend of all single-stage detectors to follow Raethers criterion, i.e. a spontaneous transition from avalanche to streamer, followed by a discharge, when the avalanche size reaches a value of a few 10 7 ; a noticeable exception is the micro-dot counter holding more than 10 8 . In multiple structures, where the gain is shared between two devices in cascade, the maximum overall gain under irradiation is increased by at least one order of magnitude; we speculate this to be a consequence of a voltage dependence of Raethers limit, larger for low operating potentials. Our conclusion is that only multiple devices can guarantee a sufficient margin of reliability for operation in harsh LHC running conditions.

Two-dimensional readout of GEM detectors

Abstract The recently introduced gas electron multiplier (GEM) permits the amplification of electrons released by ionizing radiation in a gas by factors approaching ten thousand; larger gains can be obtained combining two GEMs in cascade. We describe methods for implementing two- and three-dimensional projective localization of radiation, with sub-millimeter accuracy, making use of specially manufactured and patterned pick-up electrodes. Easy to implement and flexible in the choice of the readout geometry, the technology has the distinctive advantage of allowing all pick-up electrodes to be kept at ground potential, thus substantially improving the system simplicity and reliability. Preliminary results demonstrating the two-dimensional imaging capability of the devices are provided and discussed, as well as future perspectives of development.

Beam tests of the gas electron multiplier

Abstract We describe the results of systematic measurements, carried out with single and double GEM detectors with printed circuit read-out and having an active area of 10×10xa0cm2, both in the laboratory and in a high-energy charged particles beam at CERN. Using fast analog readout electronics, we demonstrate efficiencies for minimum ionizing particles close to 100%, with typical signal/noise ratios above 50 and up to 103 for the single and double GEM configuration, respectively, and a time resolution of 15xa0ns fwhm. Localization accuracies around 40xa0μm r.m.s. have been obtained for perpendicular tracks, degrading to 200xa0μm at 20° of incidence to the normal. Operated in a non-flammable gas mixture (argon–carbon dioxide), GEM detectors are robust, light and cheap to manufacture, and offer excellent performances and reliability suited for use in the harsh environments met at high luminosity colliders.

Further developments and beam tests of the gas electron multiplier (GEM)

Abstract We describe the development and operation of the Gas Electron Multiplier (GEM), a thin insulating foil metal-clad on both sides and perforated by a regular pattern of small holes. The mesh can be incorporated into the gas volume of an active detector to provide a first amplification channel for electrons, or used as stand alone. We report on the basic properties of GEMs manufactured with different geometries and operated in several gas mixtures as well as on their long-term stability after accumulation of charge equivalent to several years of operation in high-luminosity experiments. Optimized GEMs reach gains close to 10u2008000 at safe operating voltages, permitting the detection of ionizing tracks, without other amplifying elements, on a simple Printed Circuit Board (PCB), opening new possibilities for detector design.

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.

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.

High gain operation of GEM in pure argon

Abstract We study the operation of the Gas Electron Multiplier (GEM) in pure Ar, in comparison to that in Ar–CO2 mixture. In pure Ar, high GEM gains, of above 700 and 3000 for single and double GEM structures, respectively, have been obtained. It is observed that the GEM effective gain and its charging-up are strongly affected by electric field values above and below the GEM. Applications to the development of non-ageing gas photomultiplier are discussed.

GEM OPERATION IN PURE NOBLE GASES AND THE AVALANCHE CONFINEMENT

Abstract We study the operation of the Gas Electron Multiplier (GEM) in pure Ar and almost pure Xe. Rather high gas gains obtained in pure Ar, of the order of 1000, are explained by the effect of the avalanche confinement to a GEM micro-hole. Applications to the development of non-ageing sealed photon detector filled with pure noble gases are discussed. In particular, it is shown that the photoelectron collection efficiency deteriorated in pure Ar due to electron backscattering, can be recovered by operation at a higher electric field.

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.