F. Sauli

GEM: A new concept for electron amplification in gas detectors

We introduce the gas electrons multiplier (GEM), a composite grid consisting of two metal layers separated by a thin insulator, etched with a regular matrix of open channels. A GEM grid with the electrodes kept at a suitable difference of potential, inserted in a gas detector on the path of drifting electrons, allows to pre-amplify the charge drifting through the channels. Coupled to other devices, multiwire or microstrip chambers, it permits to obtain higher gains, or to operate in less critical conditions. The separation of sensitive and detection volumes offers other advantages: a built-in delay, a strong suppression of photon feedback. Applications are foreseen in high rate tracking and Cherenkov Ring Imaging detectors. Multiple GEM grids assembled in the same gas volume allow to obtain large effective amplification factors in a succession of steps.

Charge amplification and transfer processes in the gas electron multiplier

Abstract We report the results of systematic investigations on the operating properties of detectors based on the gas electron multiplier (GEM). The dependence of gain and charge collection efficiency on the external fields has been studied in a range of values for the hole diameter and pitch. The collection efficiency of ionization electrons into the multiplier, after an initial increase, reaches a plateau extending to higher values of drift field the larger the GEM voltage and its optical transparency. The effective gain, fraction of electrons collected by an electrode following the multiplier, increases almost linearly with the collection field, until entering a steeper parallel plate multiplication regime. The maximum effective gain attainable increases with the reduction in the hole diameter, stabilizing to a constant value at a diameter approximately corresponding to the foil thickness. Charge transfer properties appear to depend only on ratios of fields outside and within the channels, with no interaction between the external fields. With proper design, GEM detectors can be optimized to satisfy a wide range of experimental requirements: tracking of minimum ionizing particles, good electron collection with small distortions in high magnetic fields, improved multi-track resolution and strong ion feedback suppression in large volume and time-projection chambers.

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.

Investigation of some properties of multiwire proportional chambers

Abstract This article describes a systematic study of the efficiency, space resolution and multiparticle separation in multiwire proportional chambers. For a variety of gas mixtures, results were obtained as functions of high voltage, time resolution and incidence angle of particles. A new gas, argon+isobutane+freon-13 B1 permits a considerable gain in amplification without entering into the Geiger or spark region.

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.

The multistep avalanche chamber: A new family of fast, high-rate particle detectors

Abstract We have investigated an unusual charge-multiplication mechanism in gases, mainly photon-mediated, that allows a controlled avalanche spread in a parallel-plate chamber. Exploiting this mode of operation, electrons can be multiplied and transferred through a succession of amplifying elements, thus constituting a multistep avalanche chamber. Several detection schemes are analysed, which provide remarkable energy, time, and position resolution both for soft X-rays and for charged particles. A gated operation of the device is described, which should allow efficient detection of particles fluxes some orders of magnitude larger than the conventional multiwire proportional chambers. Applications in several domains, such as Cherenkov ring imaging, detection of thermal neutrons, and radio-chromatography, are also discussed.

The multistep avalanche chamber: A new high-rate, high-accuracy gaseous detector

Abstract A new particle detector relying on an unusual avalanche mechanism mainly mediated by UV photons permits multistage amplification of ionization electrons. Single electrons are detectable. Particle fluxes, orders of magnitude more intense than in wire chambers, are acceptable. Applications can be foreseen for Cerenkov light imaging, radio-chromatography, slow neutron and X-ray detection.

Progress in high-accuracy proportional chambers

Abstract For ionizing events punctually localized in the volume of a proportional chamber, the mean position of the corresponding avalanches are well defined. By reading the position of the ion cloud, using pulses induced in cathode strips arising from absorption of low-energy X-rays, the position accuracies obtained are: along the anode wire σ ∼ 35 μ m, and on the direction orthogonal σ ∼ 150 μ m with 2 mm wire spacing. The intrinsic position accuracy of the method of measurement is much better.

New observations with the gas electron multiplier (GEM)

Abstract We describe recent measurements realized with the Gas Electron Multiplier (GEM) mesh added as pre-amplification element to a multiwire and a micro-strip chamber. Large, stable combined gains are obtained, with good uniformity and energy resolution, in a wide range of filling gases including non-flammable mixtures; coupled to a micro-strip plate, the pre-amplification element allows the detector to maintain the high-rate capability and resolution at considerably lower operating voltages, completely eliminating discharge problems. Charge gains are large enough to allow detection of signals in the ionization mode on the last element, permitting the use of a simple printed circuit as read-out electrode; two-dimensional read out can then be easily implemented. The absence of charge multiplication in the last stage avoids charge build-up on the substrate and prevents ageing phenomena. A new generation of simple, reliable and cheap fast position-sensitive detectors seems at hand.