Photoelasticity for the investigation of internal stress in BGO scintillating crystals

Quality control of scintillating crystals production requires the on-line monitoring of the state of internal stress levels. The determination of the photoelastic constant in crystals is fundamental in this measurement, rendered necessary by the crystal fragility, a limiting factor in the construction of calorimeters (radiation detectors consisting in large arrays of crystals). In this work high-density EGO (Bismuth Germanate Bi3Ge4O12) crystals for calorimeters have been studied by means of photoelasticity. A test bench, based on a circular polariscope, an image acquisition system, a data processing unit and a suitable mechanical loading unit have been developed and applied to crystal sample testing. The results provide new data on photoelasticity of EGO and outline the basic features for the construction of an apparatus for on-line inspection of crystal quality.