Development of high-resolution muon tracking systems based on micropattern detectors

Abstract

A muon tracking system consisting of four 9 cm×10cm sized bulk Micromegas detectors with 128μm amplification-gap and two 10cm×10cm triple GEM detectors is foreseen for high-precision tracking of 140 GeV muons at the H8 beamline at CERN with a rate of up to 10 kHz and an overall resolution below 40μm. Larger detectors with an active area of 0.5 m2 and more are under development for detector studies in high neutron or gamma ray background environments at the Gamma Irradiation Facility at CERN and the Munich tandem accelerator. Signal studies of both detector types have been performed by recording cosmic muon and 5.9 keV X-ray signals with a single charge-sensitive preamplifier using several gas-mixtures of Ar:CO 2 . The signals were digitized using 1GHz VME based flashADCs with 2520 sampling points. The analysis of the complete signal-cycles allows for the determination of rise times, pulse heights, timing fluctuations and discrimination of background, resulting in a FWHM energy resolution of about 20% and detection efficiencies of 99% and more. Models for signal formation in both detector types will be presented. The single detector spatial resolution of 80μm was measured using a fast Gassiplex based strip readout with readout strips of 150μm width and a pitch of 250μm. The Gassiplex readout, formerly used at the HERMES experiment, had to be substantially adapted. No more crosstalk or non-linearities were observed after reconfiguration of the multiplexing amplifier on the frontend boards. The observed spatial resolution is limited by multiple scattering of the cosmic muons used in the laboratory. We also report on the sensitivity to gamma- and neutron background and on the behaviour of spatial resolution as a function of background rates.