M. Bozzo

The micro-groove detector

We introduce the Micro-Groove Detector (MGD), a new type of two-dimensional position-sensitive gas proportional counter produced using advanced Printed Circuit Board (PCB) technology. The MGD is based on a thin kapton foil, clad with gold-plated copper on both sides. An array of micro-strips at a typical pitch of 200 lm is defined on the top metal layer. Using as a protection mask the metal left after the patterning, charge amplifying micro-grooves are etched into the kapton layer. These end on a second micro-strip pattern defined on the bottom metal plane. The two arrays of micro-strips can have an arbitrary relative orientation and so can be used for read-out to obtain 2-D positional information. First results from our systematic assessment of this device are reported: gas gain ’15 000, rate capability above 106 mm~2 s~1, energy resolution 22% at 5.4 keV, no significant charging or aging e⁄ects up to 5 mC/cm and full primary charge collection eƒciency even at high drift fields. ( 1999 Published by Elsevier Science B.V. All rights reserved.

A large area, high gain Micro Gap Chamber

Abstract A new approach to the construction of the Micro Gap Chamber is presented. A 10 × 10 cm 2 MGC has been built using a 8 μm thick polyimide layer as anode-cathode insulator. Studies on gas gain, uniformity of response along the strip and charging-up have been carried out in laboratory by using X-ray sources. Very large proportional gains, up to ∼ 210 4 , have been reached working with gas mixtures based on Ne-DME. The simplified technology for the detector fabrication opens the possibility to produce very large area MGCs.

Development of a very large area microstrip gas chamber for the CMS central tracking system

A very large area microstrip gas chamber (250 × 100 mm2), thought to be the building block of the CMS barrel tracker, has been built and undergone extensive tests with X-ray sources and particle beams. Rate capability, uniformity of response along the strip and charging-up effect for different gas gain and bias schemes have been measured in laboratory. A systematic study on the long-term stability of its performance (ageing) at high rates has been carried out both with standard and “clean” mechanical assemblies. Stability measurements under high voltage were performed. Studies of spatial resolution and efficiency for minimum ionizing particles were carried out with different gas gap, gas mixture, angle of incidence and avalanche gain.

FURTHER TEST AND DEVELOPMENT OF THE MICRO-GAP CHAMBER

Abstract Results from a beam test of the micro-gap chamber with minimum ionizing particles are reported. The dependence of the spatial resolution on the incidence angle of the beam with respect to the detector plane has been studied. Laboratory tests on new amplification structures, designed to increase gain and stability and to reduce the detector capacitance, are also shown. A maximum gas gain of 6000 and a specific capacitance of 0.5 pF/cm have been obtained. An optimised layout of the MGCs structure to avoid edge effects is discussed.

The CMS micro-strip gas chamber project - development of a high-resolution tracking detector for harsh radiation environments

Abstract Thirty-two large-area Micro-Strip Gas Chambers were tested in a high-intensity, 350 MeV pion beam at PSI to prove that we had reached a Milestone for the Compact Muon Solenoid (CMS) experiment. The particle rate was approximately 6 kHz / mm 2 , distributed over the whole active area of the detectors, and this rate was maintained for a total integrated time of 493 h. All of the chambers were operated with signal-to-noise values at or above that corresponding to 98% hit detection efficiency at CMS; the average S / N was 31. No indications of any gain instabilities or ageing effects were observed. In the official 3-week Milestone period, three strips from a total of 16xa0384 were damaged, a result which is 20 times lower than the minimal requirement for CMS. The spark rate of the detectors was very low and decreased with time to an average of one spark per chamber per day. The cathode voltages of 24 of the chambers were increased over a one week period to investigate the behaviour of the detectors at higher gains; the maximum S / N value was 2.4 times that at the normal working point. No significant increase in spark rate or strip loss rate was detected and the chambers operated stably. The detector efficiencies and imaging capabilities were also investigated. The MSGC design features and the assembly and test methodologies that enabled us to achieve these results are reported.

A UV light photo-detector based on a MicroGap Chamber with single electron response

Abstract A MicroGap Chamber (MGC) used as a fast UV light detector with single electron sensitivity is presented. The detector has a semitransparent or reflective CsI photocathode, 100–200 μm anode pitch and 2D capability. It works with gas mixtures based on DME and light noble gases at atmospheric pressure. A gas gain >106 has been achieved with a two-stage gas amplification (G > 102 in the drift region and G ≈ 104 in the strip region). The use of light noble gases, like neon, has brought to a sensible reduction of VUV light output (hence photon feedback) during electron multiplication in gas. Due to the very thin gap (5 μm) a very large fraction of the avalanche charge is quickly delivered to the fast amplifier (20 ns peaking time) thus improving the single electron detection capability of the MGC. Because of its higher gain the MGC with a semitransparent CsI photocathode provides a better single electron detection efficiency (>90%) than the MGC with a reflective photocathode. This latter, vice versa, has a higher quantum efficiency and a faster response. Experimental results on gas gain, drift properties, positional sensitivity and single electron detection efficiency are reported.

Technique for the characterization of discharges in micro-strip gas chambers

A method is described for the characterization and counting of discharges in MSGCs. It is shown that the technique is sensitive enough to detect the micro-discharges which occur prior to full sparking, with a detection efficiency of 0.8. Some example results are presented, showing the variation of micro-discharge rate with gain.

A two-stage, high gain micro-strip detector

Abstract A two stage position-sensitive gas proportional counter has been constructed by tightly coupling a Gas Electron Multiplier (GEM) with a Micro-Groove Detector (MGD). The GEM was used as the first amplifying stage and was optimised to transmit close to 100% of the primary charge even at very high drift fields (10xa0kV/cm). Very narrow GEM-MGD separations (0–600xa0μm) were used so that the active volume of the detector is still very thin (3–3.6xa0mm) and the required drift field could be maintained using an acceptable drift voltage (around 4000xa0V). Very high combined gains (up to 3×10 5 ) were obtained with this system. The detector was found to be spark-free in the presence of HIPs (alpha particles) up to gains in excess of 20xa0000.

Performance of a prototype of the CMS central detector

A prototype of the barrel Tracking Detector of the Compact Muon Solenoid (CMS) experiment proposed for LHC was built and tested in a beam and in a magnetic field of up to 3 T. It contained six microstrip gas chambers, 25 cm long, and three double-sided silicon microstrip detectors, 12.5 cm long. We report some preliminary results on the performance of the chambers.

THE MICRO-GAP CHAMBER : NEW DEVELOPMENTS

To fully exploit the capabilities of the Micro-Gap Chamber, new geometries are examined in the light of the recent work performed in our labs. Results on the usage of neon in the gas mixtures necessary to operate the chamber at high gas gain and of ageing tests with these new mixtures are also discussed.