Two muon walls are foreseen for the COMPASS detector which are dedicated to different physics objectives.
The first muon wall is mainly needed for the detection of low energy muons (up to 25 GeV) from the semileptonic decays of charmed baryons. Less than 10% of muons from deep inelastic scattering with y>0.5 will hit this wall. The first two muon stations of the first muon wall will be placed after the hadron calorimeter HCAL1, followed by 1 m of iron and two more muon stations. The multiple scattering of low energy muons in the absorber is quite high (about 12 mrad for 10 GeV muons) and the demands on the detector resolution are thus quite modest. The spatial resolution needed to match the effect from the multiple scattering in the iron is about 5 mm. In addition we have to consider the scattering in the hadron calorimeter which is relevant to match the downstream muon track with the upstream one. We assume the multiple scattering to be again about 12 mrad at 10 GeV and arrive at a needed spatial resolution of about 0.5-1 cm.
The second muon wall is dedicated to the detection of
the high energy muons in the deep inelastic scattering program and the
high momentum part of the decay muons from charmed hadrons.
The muon detectors will be located after 2 m of iron
downstream of HCAL2.
The multiple scattering of 200 GeV muons in 2 m of iron is about 0.7 mrad.
The detector resolution should match this value. Using two
stations of the detector with spatial resolution of 300 
m each
with distance of 50 cm between them meets this requirement.
The background rate for the detectors from the muon halo is 50-180
.