G. Million

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.

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.

Performance of gas microstrip chambers on glass substrata with electronic conductivity

Abstract We present some recent measurements on Gas Microstrip Chambers (GMSC) realised on glass substrata with different bulk resistivity. Chambers manufactured on low resistivity supports (∼ 10 9 Ohm·cm) have very high rate capability, up to and above 5×10 6 counts mm −2 ·s −1 at proportional gains around 10 3 . With increase of resistivity of the support, the rate capability decreases, presumably because of the modification of electric field induced by the current flowing along the resistive surface, and at ∼10 12 Ohm·cm the gain is affected already at the level of ∼5×10 5 counts mm −2 ·s −1 . Long term measurements of gain show a stable behaviour for the chambers built on low resistivity glass, up to an integral charge of ∼130 mC per cm of strip. At normal operating conditions (an avalanche size of about 10 5 electrons) this corresponds to 10 Mrad of adsorbed dose for minimum ionizing particles, thus making the GMSC a good candidate for the use in LHC and SSC detectors. In order to reach the quoted lifetime, particular precautions have to be taken to guarantee gas purity. The present study has been realised in the framework of the collaboration for the Development of GMSC for Radiation Detection and Tracking at High Rates (CERN Research and Development Project RD28).

Optimization of microstrip gas chamber design and operating conditions

Abstract We discuss recent experimental and theoretical work aimed at optimizing the structure and operating conditions of microstrip gas chambers. In a systematic set of measurements we have found gas mixtures allowing high stable gains; the best results were obtained using argon-dimethylether in equal percentages (50-50), and mixtures of dimethylether and carbon dioxide. Detectors exhibit lower noise and better energy resolution when using high drift fields, i.e. above 5 kV/cm; in these conditions, stable gains in excess of 104 could be achieved. We also discuss a model to characterize the breakdown depending on the combined field strength at the edges of the anode and cathode strips; using a program that computes fields and gains and applying the discharge criterion, we have estimated the optimum geometry of the detectors. The results indicate that higher gains before discharges can be obtained for wide cathodes: as an example, for 10 μm anodes we predict an increase by a factor of 4 of the maximum gain when increasing the width of the cathode strips from 80 μm to 160 μm.

Progress in Cherenkov ring imaging: Part 1. Detection and localization of photons with the multistep proportional chamber

Abstract The multistep proportional chamber, operated with a photosensitive gas filling, makes it possible to obtain stable multiplication factors in excess of 106 and can be used for the detection of single photoelectrons released in the gas. The efficiency and localization properties of the device in the detection of vacuum ultraviolet photons are discussed here, in view of its use for particle identification exploiting the Cherenkov ring-imaging method.

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.

Ageing studies with microstrip gas chambers

Abstract Microstrip gas chambers (MSGCs) are intrinsically capable of operating at very high radiation fluxes, up to and above 106 particles mm-2 s-1. It was noticed, however rather early in the development of the detectors that sustained exposure to radiation could produce irreversible damage (decrease of gain and discharges) easily assimilated with the ageing phenomena which are known to occur in proportional counters, and caused by the polymerization of gas molecules in the avalanche process. We describe the results of a systematic investigation of ageing under sustained irradiation of MSGC plates manufactured on several substrates and operated in clean laboratory containers as well as in mechanical assemblies better suited for the use of the detectors in experimental set-ups. In the best experimental conditions, we have demonstrated survival without degradation of the detectors up to a collected charge of 120 mC cm-1; in less than optimal conditions, ageing is observed already at a level of a few μC cm-1. A careful selection of the operating gas and of the materials used in manufacturing appears mandatory to guarantee survival of the devices in a high radiation environment such as that of the Large Hadron Collider (LHC) detectors. The present study has been realized at CERN in the framework of the RD-28 Collaboration (Development of microstrip gas chambers for high-rate applications).

Performance of MSGC on electronically and ionically conductive substrata in various operational conditions

Abstract Recent measurements realized with Microstrip Gas Chambers (MSGC) manufactured on glass substrata in a range of bulk resistivity are presented. Chambers made on electronically conductive supports with resistivity of 10 9 ω cm and gold strips show a stable behaviour during long term irradiation up to an accumulated charge of 130 mC/cm of strip when operated in an argon-dimethylether (DME) mixture. Similar measurements realised with argon-methane show instead a fast degradation of gain already at collected charge of around 5 mC/cm. Further investigation of MSGC performance with different Ar-DME compositions show that the larger stable gain can be achieved with a 50-50 mixture. We present also results of beam runs performed with MSGC manufactured on Hoya SL glass with an anode to cathode distance of 200 μm and a 2 mm gas layer. The dependence of efficiency and spatial resolution on the incidence angle of track was measured with argon-DME and xenon-DME gas mixtures; for minimum ionizing particles perpendicular to the chambers (0°) the spatial resolution for Xe gas mixture is 65 μm rms and the efficiency 98%. For incidence angles up to 10° a small degradation of both parameters was observed.