Nuclear Inst~menu and Methods in Physics Research A 368 ( 1995) 259-264 zyxwvutsrqponmlkjihgfedcbaZYXWVU NUCLMR INSTRUMENTS (L METNODS IN PNYSICS RESEARCH Section A ELSEVIER The MicroGap Chamber: a new detector for the next generation of high energy, high rate experiments R. Bellazzini *, G. Spandre zyxwvutsrqponmlkjihgfedcbaZYXWVUT INFN-Piss and University zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJI of Ph. Piss. Itaiy Abstract The concept of MicroGap Chamber (MGC) is introduced. Results from a large MGC ( 10 x 10 cm2) with a small stereo angle read-out are presented. Both coordinates are read out from the same side of the detector substrate. Measurements of gain stability, fraction of induced charge on the back electrode, uniformity of response along the strip and rate capability have been performed in the laboratory, using X-ray sources. Detection efficiency, spatial resolution, charge and space correlation have been measured with minimum ionizing particle beams at CERN. 1. Introduction The MicroGap Chamber (MGC) [l] is a new type of position sensitive proportional gas counter made with mi- croelectronics technology. It can be thought of as an ultra- miniaturized version of the MultiWire Proportional Cham- ber (MWPC), but in a configuration of highly asymmetric anode-cathode gaps. One half-gap has a standard thickness of 2-5 mm, the other has a thickness of only S-10 pm. The thick gap provides the primary ionization charge, while the ultrathin one is used to very quickly collect the positive charge created during the avalanche process. While in the MWPC the anodes float in the gas, in the MGC they are stuck onto a very thin insulating strip only a few microns wider than the anode microstrip itself. The introduction of the microelectronics technology instead of the iron solder technology in the design and manufacturing of radiation de- tectors allows the anode pitch to be reduced from the few millimeters typical of the MWPC down to 100-200 pm, and to reach a spatial resolution of a few tens of microns (20-40 pm). Peculiar to the structure of the MGC is the separation of only a few microns between the anode and the positive-ion- collecting cathode. Due to the large number of field lines ending on the very close back-cathode and to the steep volt- age gradient, the process of avalanche charge collection is very fast. About 80% of the total charge is delivered to the preamplifier in 10 ns. In the same time, an almost equal amount of charge is induced on the back-cathode strips. As a result, the rate capability is extremely high (-10’ counts/mm2 s) . The speed of this device is now very close * E-mail bella.zzini@pisa.infn.it Else&x Science B.V. SSDIO168-9002(95)01273-7 to that of solid state detectors. Furthermore, the high elec- tric field which extends over a very small region is not much affected by the anode pitch. This permits an easy design of a detector with variable pitch. The amplifying electric field around the thin-anode microstrip extends over a small vol- ume, but is very intense. It provides gas gains > IO3 at 400 V with 14% (FWHM) energy resolution at 5.4 keV. Another interesting characteristic of the device is its in- trinsic 2-dimensional nature. The back-cathode can be suit- ably structured for the reconstruction of a coordinate not necessarily orthogonal to the anode strips. Radial, stereo or pads structures can be chosen as well. Due to the good space and time resolution, high rate capa- bility, 2-D capability and considerable flexibility, this device seems very well suited for the instrumentation of tracking systems. It fulfills almost all the stringent requirements of the next generation of experiments (CMS, ATLAS, HERA-B, LHC-B, ALICE) [2-4]. 2. Tire detector The MGC can be considered as the 3-dimensional evo- lution of the MicroStrip Gas Chamber (MSGC) [ 51) The amplifying electrode structure is no longer in just one plane; use is also made of the third dimension. The anode and cath- ode are now in two different planes with the insulation be- tween them provided by a thin insulating strip. They can be stmctured independently, if so desired. A schematic cross-section of a 2-D MGC is shown in Fig. la (not to scale). The figure refers to the so-called “self-aligned” amplification structure [