C. Garabatos

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.

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.

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.

Optimization of design and beam test of microstrip gas chambers

Abstract We describe recent experimental and theoretical work aimed at optimizing the geometry and the operation of micro-strip gas chambers in order to improve their performance and reliability. With the help of a simulation program, we have studied the mechanism of signal propagation and analyzed the effects on signal shape and size of resistivity of strips, grouping of biased strips and the presence of a back-plane. Several detectors manufactured according to the results of the study and equipped with fast amplifiers have been installed in a test beam to study general operating characteristics, efficiency and localization accuracy; preliminary results of the data analysis are discussed.

Aging properties of straw proportional tubes with a XeCO2CF4 gas mixture

Abstract Studies of the behaviour of straw proportional tubes, 4 mm in diameter, under sustained irradiation were carried out with a XeCO 2 CF 4 [50-30-20] gas mixture. No observable gain drop has been found to affect the tubes up to reasonable doses ( ≤ 1 C/cm of wire). The chemical modification of the gas composition, causing the formation of electronegative, relatively stable species during the avalanche processes in a highly irradiated straw tube, is suggested to be responsible for a decrease of the pulse height in a second proportional counter (12.5 mm drift distance) operated with this (radical-enriched) gas mixture. Finally, we have observed that, for a straw tube operated at gain of a few times 10 4 , there is a contribution to the total collected charge coming from few, very large pulses, probably due to occasional transition to a limited streamer regime.