First beam test on a BSO electromagnetic calorimeter

A beam test has been performed on a prototype electromagnetic calorimeter of bismuth silicate BSO crystals for the first time using electrons of 0.5-3.0 GeV. The calorimeter consists of nine BSO crystals, 22mm×22mm×180mm (15.6 radiation lengths), arranged in a 3×3 array. Each crystal is coupled to a small photomultiplier tube of 20 mm in diameter. The obtained energy resolution is σE/E=(0.023±0.003)/E(GeV)⊕(0.017±0.002) for incident electrons of the energy E in GeV. The position information for an electron injected onto the center crystal of the calorimeter array is given with energy deposit signals from nine BSO crystals. The RMS position resolution σx is parameterized as σx=[(3.2±0.1)/E(GeV)⊕(1.3±0.1)]mm at the center of the crystal. Misidentification probability of pions to electrons is found to be ∼10-3 for the pion momentum ranging from 1.0 to 3.0 GeV/c.