Development of a 150 m$^{2}$ proportional chamber system with a 1 million bit buffer: the EMI for BEBC

For the identification of muons behind the Big European Bubble Chamber (BEBC) a detector wall of 6 m × 25 m is required. We describe the construction of the wire chambers, the electronics and the performance of the first chambers. The 50 modular 3 m × 1 m chambers contain 75 000 wires and 10 000 cathode strips, grouped into 20 000 electronic channels to give a variable space resolution. Geometrical acceptance is maximized by the suppression of support lines and spacers. The electronics can buffer up to 1023 events without dead time losses and without limitation on the number of hits per event. Ar/CO 2 mixtures have given good results.