Performance of a BGO calorimeter below 1.6 GeV

Abstract An array of 7 bismuth germanate (BGO) modules, each being 17 or 20 cm long and having a hexagonal cross section with 50 mm across the parallel edges, has been tested with 0.2–1.6 GeV/ c electrons and pions. The fwhm energy resolution for 1 GeV electrons was 4.1% in agreement with the EGS calculations. The e/π separation for momentum analyzed particles at 1 GeV/ c was 1 500 with only the energy cut, and was improved to 1 2000 with an additional cut in the lateral energy spread. Finally a calorimeter has been constructed of 10 BGO modules surrounded with NaI slabs and successfully employed in KEK Experiment E68 to measure the γ ray energy spectrum in p p annihilation at rest. A fwhm resolution of 10% was obtained for 129 MeV γ rays.