A study of the light output and energy resolution of small BGO crystals

High light output and excellent spatial and energy resolution are important for scintillation crystals used in high resolution positron emission tomography (PET). The authors have studied some factors influencing these parameters. Two identical 3×5×30 mm bismuth germanate (BGO) crystals, one polished and the other rough, were subjected to 3 surface treatments: wrapping with white teflon tape, coating with barium sulphate paint and coating with white silicone caulking. The results were compared with those for crystals with no surface treatment. The best results were obtained with a polished BGO crystal using teflon tape as reflector. The energy resolution of the rough crystal showed a marked depth of interaction dependence. The authors studied the effects of grinding the faces parallel to the PMT face of a polished BGO crystal. The best energy resolution and the highest light output were obtained for the crystal with the face coupled to the PMT roughened and all remaining sides polished and covered with teflon tape. The authors have also studied the response of a 15×10×20 mm block of BGO with 0.15 mm wide and 8 mm deep slits, 2 mm apart in 2 directions on one end. The cut block had energy resolution a factor of 2 worse than the energy resolution of the solid crystal. The light output of the cut part varied depending upon the face coupled to the PMT. The authors have evaluated different methods for improving the resolution and light output of block crystals. The most promising results were obtained when the block was etched for 24 hours with 27N (75%) sulphuric acid. After etching, the light output of the block increased by 20%