Geiger Mode Calorimeter for PEP-4

The PEP-4 hexagonal barrel calorimeter, with lead-laminate layers and 5×lOmm2 Geiger-mode discharge cells, has demonstrated excellent stability and uniformity in operation. The use of projective geometry in half degree-wide cathode strips at ±60 degrees to the sense wire channels provides excellent spatial resolution and reconstruction capability. The electronic noise-to-signal ratio without preamplification is sufficiently low that individual 50 pC Geiger discharges are accurately measured and used for energy calibration. Measurements made at 14.5 GeV e± beam energy have provided preliminary spectra of Bhabhas and of photon-photon invariant mass. The latter show that ?o's can be reconstructed.