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 4 π 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 π 0 's in jet events is in excellent agreement with Monte Carlo simulations. The π 0 mass resolution is MeV (rms).