BABAR CsI calorimeter design and first beam test results

The BABAR detector at the PEP-II B-factory at SLAC is designed to measure CP violation in the decays of B mesons. In order to meet the physics aims of the experiment the target energy resolution for photons at a polar angle of 90° is: σE/E=(1%/4E [GeV]) ⊕ 1.2%. This requires a minimum equivalent noise energy of about 150 keV per crystal. Using two 1 × 2 cm2 photodiodes for direct readout of full-size crystals a photoelectron yield of about 8000 e−/MeV and equivalent noise energies below 100 keV were obtained in the laboratory. t beam test using a 5 × 5 array of crystals was performed in the fall of 1995 at an electron/pion beam at PSI. Data were collected at different energies between 100 MeV and 400 MeV allowing the determination of energy and angular resolution as a function of energy, impact position and angle of incidence. Results of a first analysis of these data are presented.