Study of a large BGO crystal in a charged particle beam

Two large crystals of bismuth germanate (BGO) were studied with sources and in a pion beam. The response and uniformity have been investigated with several types of reflectors. The temperature dependence of the emitted light was determined, as well as the timing resolution. As the crystal is intended to be part of a large array with very good energy resolution in the detection of high-energy gamma rays and electrons, uniformities of better than 0.5% are required. Various methods to achieve this are discussed. Results from beam and source measurements show that the relative root-mean-square deviation of the output peak positions for the detection of gamma-rays interacting at different regions in a 24-cm-long truncated pyramid BGO crystal with MgO reflectance coating covered by aluminized mylar and black tape is about 1%