Quality Control and Structural Assessment of Anisotropic Scintillating Crystals

Nowadays, radiation detectors based on scintillating crystals are used in many different fields of science like medicine, aerospace, high energy physic and security. The scintillating crystals are the core elements of these devices; by converting high energy radiations in visible photons, they produce the optical signals to be detected and analyzed. Structural and surface conditions, defects and residual stress states play a crucial role in their operating performances like light production, transport and extraction. Industrial production of such crystalline materials is a complex process which requires sensing, in-line and off-line, for material characterization and process control to properly tune the production parameters. Indeed, scintillators quality must be accurately assessed during their manufacturing in order to prevent malfunction and failures at each level of the chain, optimizing the production and utilization costs. This paper presents an overview of the techniques used, at various stages, across the crystal production process, to assess quality and structural condition of anisotropic scintillating crystals. Different inspection techniques (XRD, SEM, EDX, TEM) and the non-invasive photoelasticity based methods for residual stress detection, such as Laser Conoscopy and Sphenoscopy, are presented. The use of XRD, SEM, EDX and TEM analytical methods offer detailed structural and morphological information. Conoscopy and Sphenoscopy offer the advantages of fast and non-invasive measurement methods suitable for the inspection of the whole crystal quality. These techniques, based on different measurement methods and models, provide different information which can be cross-correlated to obtain a complete characterization of the scintillating crystals. Inspection methods will be analyzed and compared in relation to present state of the art.