Ph. Rebourgeard

MICROMEGAS: a high-granularity position-sensitive gaseous detector for high particle-flux environments

Abstract We describe a novel structure for a gaseous detector that is under development at Saclay. It consists of a two-stage parallet-plate avalanche chamber of small amplification gap (100 μm) combined with a conversion-drift space. It follows a fast removal of positive ions produced during the avalanche development. Fast signals (≤1 ns) are obtained during the collection of the electron avalanche on the anode microstrip plane. The positive ion signal has a duration of 100 ns. The fast evacuation of positive ions combined with the high granularity of the detector provide a high rate capability. Gas gains of up to 10 5 have been achieved.

Micromegas as a large microstrip detector for the COMPASS experiment

Abstract Recent results on the gaseous microstrip detector Micromegas which will be used to track particles in the COMPASS experiment at CERN are presented. Developments concerning its mechanical and electrical design, associated readout electronics and gas mixture were carried out. Particular attention was paid to the discharge phenomenon which affects this type of microstrip detector. The adequacy of the options finally retained, especially the SFE16 readout and the use of a Ne–C 2 H 6 –CF 4 gas mixture, was demonstrated in a set of beam tests performed on a 26×36 cm 2 prototype. Operating at a gain of ∼6400, full efficiency is reached along with a spatial resolution of ∼50 μm and a timing accuracy of 8.5 ns . Discharges are kept at a low rate, less than one per SPS spill in a COMPASS-like environment. Via a decoupling of the strips through individual capacitors their impact is greatly reduced. They generate a dead time on the full detector of ∼ 3 ms , affecting marginally the detection efficiency given their rate. The probability of discharge, at a given value of efficiency, is found to decrease with the mean value of the gas mixture atomic number. In view of these results, the commissioning of Micromegas for COMPASS is foreseen in the near future.

Fast signals and single electron detection with a MICROMEGAS photodetector

Abstract The performance of a new gaseous photodetector was investigated. It consists of a solid photocathode and a gas amplification structure of the MICROMEGAS type. Using a mixture of helium and isobutane at atmospheric pressure, a stable and high amplification gain close to 107 was achieved. Such a high gain and small fluctuations allowed the detection of single photoelectrons with a time resolution better than 700 ps. These performances are comparable with those obtained with the best photomultipliers.

New developments of Micromegas detector

Abstract A new type of micromesh, based on etching techniques, has been developed for the Micromegas detector. In this paper, we will briefly describe this new design and give some results about the performances obtained in different gas mixtures. The geometry of the mesh allows good uniformity of the electrostatic field. An energy resolution of 11.7xa0% full-width at half-maximum is obtained with X-rays at 5.9xa0keV and 5.4xa0% at 22xa0keV in an argon/isobutane (90%/10%) gas mixture. This is a significant improvement for a gaseous detector operating at high gain (about 5000).

First beam test results with Micromegas, a high-rate, high-resolution detector

Abstract We present particle beam test results using a high-rate, high-position and high-time-resolution gaseous detector, ‘Micromegas’, of 15×15 cm 2 . The rate capability was measured with 10xa0MeV protons from a TANDEM accelerator. No effect on gain was observed at particle rates up to 10 9 mm −2 s −1 . With an argon and DME filling the gain was stable up to 50 mC total charge on a 3 mm 2 area. With minimum-ionizing particles in a CERN beam a high efficiency, close to 100%, was measured, under stable conditions. A first space-resolution measurement of 50 μ m ±20 μ m was obtained. The operation of the chamber shows that it is possible to optimize the geometrical parameters in order to improve the space resolution and bring the time resolution low enough to contain the events of each beam crossing (every 25xa0ns) in the European Large Hadron Collider. Further work in this direction is being actively pursued.

Development of a fast gaseous detector : "micromegas"

Abstract Several 15×15 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 5×10 5 Hz / mm 2 ) 100 GeV /c muon beam. The detector behaviour has been studied with respect to many parameters: conversion gaps of 1 and 3xa0mm, amplification gaps of 50 and 10xa0μm, an external magnetic field and many different filling gases. So far no effect of the magnetic field up to 1.3xa0T has been observed. The gas mixture argon + cyclohexane appears to be very suitable with gains above 10 5 and a full-efficiency plateau of 50xa0V at 340xa0V. With a conversion gap as small as 1xa0mm and an electronics with a threshold at 5000 electrons the efficiency reaches 96%. With the addition of CF 4 a time resolution of 5xa0ns (RMS) has been obtained. A spatial resolution better than 60 μ m has been observed with anode strips of 317 μ m pitch and was explained by transverse diffusion in the gas. Simulations show that with a pitch of 100 μ m and the appropriate gas a resolution of 10 μ 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.

Electron drift velocity measurements at high electric fields

Abstract A method to measure the electron drift velocity is presented. A pulsed UV nitrogen laser is used to excite both the drift and cathode nickel micromeshes of a Micromegas detector. The signals induced on the anode are then readout by a fast current amplifier. Several results have been obtained for various gas mixtures and electric fields from 10 V / cm to 14 kV / cm . Relevant applications with low (TPCs mode) and high (pre-amplification mode) electric fields will be discussed.

The gaseous microstrip detector micromegas for the COMPASS experiment at CERN

Abstract The measurements foreseen in the COMPASS experiment at CERN, require high resolution tracking detectors, with low radiation length and high rate capability. For this purpose we have developed and optimized a gaseous microstrip detector ‘Micromegas’. Twelve planes with 1024 strips each, assembled in 3 stations of 4 views XYUV, have been operated with success in the summer of 2002 in the COMPASS environment. We describe here the performances and results obtained.

A Fabry–Pérot cavity for Compton polarimetry

A new kind of Compton polarimeter using a resonant Fabry—Pe« rot cavity as a power buildup for the photon beam is proposed. A prototype of such a cavity is described, along with the results obtained in terms of source to be used in a Compton scattering polarimeter. ( 1998 Elsevier Science B.V. All rights reserved.

Experimental study of an impact-parameter optical discriminator

Abstract An optical discriminator, sensitive to the impact parameter has been tested in a particle beam. The device is based on the detection of Cherenkov light produced and trapped in a thin spherical crystal shell viewed by a fast photodetector. The detector is only sensitive to charged particles with a nonzero impact parameter relative to the centre of the crystal sphere. A prototype using a LiF crystal has been tested and the experimental results are in agreement with expectations. We outline the foreseen developments in order to use such a detector for a fast selection of B events in fixed-target experiments at proton accelerators. We also discuss the use of the detector as presently conceived for the fast selection of strange particles.