Performance of the hexagonal calorimeter at PEP-4

A Geiger-mode electromagnetic shower calorimeter has been constructed for PEP-4. Six trapezoidal modules cover 75% of 4π solid angle in a hexagonal array. Each module contains 40 sense wire planes between 0.25 radiation-length thick lead-laminates, and is divided electrically into submodules of 27 and 13 layers. Half-degree wide hodoscopic channels in projective geometry provide excellent spatial resolution. In each gap three stereo views using the sense wires as well as strips on both cathodes at ±60° to the wires provide good shower reconstruction capabilities in multi-track jet events. Calibration and data analysis procedures are outlined. The performance of the calorimeter in the measurement of Bhabha events and the reconstruction of π0's in jet events is in excellent agreement with Monte Carlo simulations. The π0 mass resolution is MeV (rms).