Development of a fast gaseous detector:: 'Micromegas'

Several 15 x 15 cm(2) gaseous Micromegas chambers (MICROMEsh GAseous Structure) which consist of a conversion gap and an amplification gap separated by a thin grid have been extensively tested in low-intensity 10 GeV/c pion beam and high-intensity (up to 5 x 10(5) Hz/mm(2)) 100 GeV/c muon beam. The detector behaviour has been studied with respect to many parameters: conversion gays of 1 and 3 mm, amplification gaps of 50 and 10 mu m, an external magnetic field and many different filling gases. So far no effect of the magnetic field up to 1.3 T has been observed. The gas mixture argon + cyclohexane appears to be very suitable with gains above 105 and a full-efficiency plateau of 50 V at 340 V. With a conversion gap as small as 1 mm and an electronics with a threshold at 5000 electrons the efficiency reaches 96%. With the addition of CF, a time resolution of 5 ns (RMS) has been obtained. A spatial resolution better than 60 mu m has been observed with anode strips of 317 mu m pitch and was explained by transverse diffusion in the gas. Simulations show that with a pitch of 100 mu m and the appropriate gas a resolution of 10 mu m is within reach. This development leads to a new generation of cheap position-sensitive detectors which would permit high-precision tracking or vertexing close to the interaction region, in very high-rate environments. (C) 1999 Elsevier Science B.V. All rights reserved.