Development of fast tracking detectors: micro strip gas chamber and gas electron multiplier

The work describes the preparation, realization and results of a high intensity beam test for Micro Strip Gas Chambers - MSGC and the development of a new component for gaseous detectors, the Gas Electron Multiplier - GEM, apt to overcome some problems met with MSGCs as well as a new concept. The description of MSGC beam test covers the construction, assembly and experience gained with the testing and operation of a small but complete system of high rate MSGCs. The detectors had gold and chromium strips, pitch 200μm, engraved on a thin borosilicate glass, D-263, with diamond-like coating. All were fully equipped with read out electronics (Pre Mux), which allow the recording of charge on individual anode strips. The active area of 100 x 100mm2, was exposed during six month to a high intensity muon beam. Continuous monitoring of the performance of the chambers during the beam runs allowed the evaluation of detection efficiency, monitoring of accidental rates and the study of ambient induced variation in realistic beam conditions. Efficiencies could reach up to 98 % in best operation conditions, although local lower values were often observed due to open strips, broken bonds or dead electronic channels. The long term operation of the detectors has been more difficult than expected, because of the appearance of breakdowns and loss of efficiency in some detectors. The problems were possibly induced by the presence of small gas leaks in the system, the long term exposure of the chambers component to dimethylether and the presence of heavily ionizing particles. To overcome the problems with the MSGCs a new detector component Electron Multiplier - GEM was invented, to be used as an internal charge preamplification device. In this work the manufacturing and operating principles of the GEM mesh as well as recent measurements realized with the GEM mesh added to a MSGC are described. Large, stable combined gains are obtained, with good uniformity and energy resolution and in a wide range of filling gases. Coupled to a micro strip plate, the preamplification element allows the detector to maintain the high rate capability and resolution at considerably lower operation voltages. This minimizes the problems related to the probability of the discharge. Moreover, charge gains are large enough to allow detection of signals in the ionization mode on the last element, which permits the use of a simple printed circuit as read-out electrode. Measurements with such GEM-detectors show full detection efficiency for minimal ionizing particles. (author)