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 degrees is: sigma(E)/E = (1%/(4) root/E[GeV]) + 1.2%. This requires a minimum equivalent noise energy of about 150 keV per crystal. using two 1 x 2 cm(2) 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. A first beam test using a 5 x 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.