Scintillation yield of Bi4Ge3O12 (BGO) pixel crystals

BGO crystals have been grown using the Czochralski method by the West Pomeranian University of Technology in Szczecin, Poland. We report the measurements of scintillation yield and low temperature thermoluminescence, performed on polished 2×2×10 mm3 pixel and 2×2×2 mm3 cube samples. The yield of the pixels placed horizontally on the PMT window, as well as of the cubes, is about twice higher than the yield of the pixels placed vertically. Such a distinct light loss is interpreted in the frame of a simple two-ray model, describing quantitatively the scintillation yield of pixel samples depending on the measurement geometry. Related to this effect, the common overuse of “BGO units” for expressing the light output of any scintillator is discussed. Thermoluminescence glow curves consist of several peaks attributed to the existence of traps, which turn out to be responsible for a slight decrease of the scintillation yield measured at room temperature in a short time window. To improve the performance of BGO crystals, efforts should be aimed at reducing both the internal light losses and the trap concentration, with the main emphasis laid on the former.