Quality inspection of anisotropic scintillating crystals through measurement of interferometric fringe pattern parameters

Scintillating crystals are widely used as detectors in radiographic systems, computerized axial tomography devices and in calorimeters employed in high-energy physics. This paper results from a project motivated by the development of the CMS calorimeter at CERN, which will make use of a large number of scintillating crystals. In order to prevent crystals from breaking because of internal residual stress, a quality control system based on optic inspection of interference fringe patterns was developed. The principle of measurement procedures was theoretically modelled, and then a dedicated polariscope was designed and built, in order to observe the crystals under induced stresses or to evaluate the residual internal stresses. The results are innovative and open a new perspective for scintillating crystals quality control: the photoelastic constant normal to the optic axis of the lead tungstate crystals (PbWO sub 4) was measured, and the inspection procedure developed is applicable to mass production, not only to optimize the crystal processing, but also to establish a quality inspection procedure.