Recent results on the gaseous microstrip detector Micromegas which will be
used to track particles in the COMPASS experiment at CERN are presented. De
velopments concerning its mechanical and electrical design, associated read
out electronics and gas mixture were carried out. Particular attention was
paid to the discharge phenomenon which affects this type of microstrip dete
ctor. The adequacy of the options finally retained, especially the SFE16 re
adout and the use of a Ne-C2H6-CF4 gas mixture, was demonstrated in a set o
f beam tests performed on a 26 x 36 cm(2) prototype. Operating at a gain of
similar to 6400, full efficiency is reached along with a spatial resolutio
n of similar to 50 mum 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 i
s greatly reduced. They generate a dead time on the full detector of simila
r to 3 ms, affecting marginally the detection efficiency given their rate.
The probability of discharge, at a given value of efficiency, is found to d
ecrease with the mean value of the gas mixture atomic number. In view of th
ese results, the commissioning of Micromegas for COMPASS is foreseen in the
near future. (C) 2001 Elsevier Science B.V. All rights reserved.