The Muon Veto System is made of three layers of NE-110 scintillation counters
separated by 80cm of iron. The first two planes have 1cm thick scintillators
oriented horizontally and vertically respectively. They are used in the trigger
logic to reject 
decays. The third plane has thinner (6mm)
scintillators and is mainly used to monitor the detection efficiency of the
first two planes. The efficiency of the Muon Veto System is better than
99 
for muons above 5GeV.