Performance of the external muon identifier for the Big European Bubble Chamber

Muon identification in experiments with BEBC is obtained by a combination of two planes of multiwire proportional chambers and a hadron absorber of iron (thickness up to 210 cm). The detector planes cover 18 and 150 m2, respectively, and contain some 85000 sense wires and 20000 electronic channels. This system has successfully operated in v and v experiments, the main part of it for a year. Under most severe background conditions up to 600 particles were registered in a 2 ms beam spill. The performance of the chambers, the fast electronics and the on-line data handling system is described. Ar/CO2 mixtures have proved to be very reliable. A short account is given on the next improvement stage.