Development and Performance of a Large-Scale Multilayer Proportional Counter System Used in the External Particle Identifier (EPI) at CERN

A matrix of 4096 gas proportional counters of 6 × 6 cm2 cross section and 1 m height arranged in 128 layers of 32 cells each is used for identification of fast pions, kaons and protons emerging from hadron interactions in the 3.7 m bubble chamber (BEBC). Particle separation in the momentum range up to at least 70 GeV/c is achieved by ionization sampling in the region of the relativistic rise. Resolution in dE/dx of 6% FWHM (2.5% RMS) was reached and maintained over extended running periods. The relativistic rise was measured in the momentum range of 20-110 GeV/c. The results of the EPI tests are used in analysis of certain practical application limits of the dE/dx sampling method.