Nuclear Instruments and Methods in Physics Research A 367 (1995) 168-172 zyxwvutsrqponmlkjihgfedcbaZYXWV NUCLEAR INSTRUMENTS & MET- IN PHYSICS ELSEVIER RYs” Development of micro-strip gas chambers for high rate operation R. Boucliera, M. Cape&“, C. Garabatosa, G. Manzin”, G. Million”, L. Ropelewski”, F. Sauli”‘“, L.I. Shekhtman”.‘, T. Temmel”, G. Della Meab, G. Maggionib, V. Rigatob zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML “CERN, Geneva, Switzerland “INFN, Legnaro, Italy zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML 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 10” to 10” fl cm, and boro-silicate glass with thin coatings of lead silicate with surface resistivity between 10” and lOi R/O. 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-’ 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. 1. Introduction The electrical characteristics of the substrate material used for microstrip gas chambers (MSGC) determine their high-rate behaviour. Most insulators available with suffi- ciently good surface quality have very high resistivity, above lOI 0 cm; this results in short-term and rate-depen- dent instabilities during irradiation [l]. The effects can be attributed to polarization following the application of potentials between electrodes, and to charging up of the insulator between strips due to an accumulation of elec- trons and ions produced in the avalanches. Theoretical considerations and experimental results indicate that sub- strates with bulk resistivity in the range lo9 to 10” fi cm or surface resistivity between lOI and lOI 51/O are adequate for the stabilization gain up to very high rates [2,3]. Resistivity in the mentioned range can be obtained using special electronically conducting glasses, or by appropriate surface treatments of insulators; for a review of methods see for example Refs. [4,5]. After extended exposure to ionizing radiation, MSGCs have been found to suffer from a permanent modification of gain and a deterioration of performances in the ir- ’ Present address: Budker Institute for Nuclear Physics, Novosibirsk, Russia. * Corresponding author. E-mail fabio@cemvm.cem.ch radiated area, due to the creation of thin insulating polymer layers on electrodes and substrate, followed by micro- discharges and irreversible damages. The rate of ageing appears to be extremely sensitive to the presence of trace impurities, present in the gas or released by materials used for the assembly. The nature of the support and the metal used for the electrodes seem also to play a role [3,6-lo]. Conservative estimates of the particle flux in the CMS and ATLAS tracking detectors, 40 cm from the beam pipe and 1 m from the intersect, provide a value of 2 X lo4 mm-? ss’ at a luminosity of lox4 cm-’ s- ‘. Taking into account the fraction of low momentum particles, the equivalent flux of minimum ionizing particles is 4 X 104 _-’ sm’; for an avalanche size of 10’ electrons, the current density is 0.7 nA mm-‘, a charge of 14 mC per cm of strip per effective year of LHC operation (10’ s). A reasonable lifetime requirement is therefore a negligible drop for a collected charge of -140 mC cm- ’ 2. Experimental set-up All MSGCs used in the present study have been manufactured with similar geometry: a strip width of 90 km for cathodes and 7 km for anodes, with a pitch (distance anode-to-cathode centres) of 100 km and an active area of 80 X 82 mm*. Cathode strips are individually 0168~9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00576-S