Abstract
Nowadays, radiation detectors based on scintillating crystals are used in many different fields of science like medicine, aerospace, high-energy physics, and security. The scintillating crystals are the core elements of these devices; by converting high-energy radiation into visible photons, they produce optical signals that can be detected and analyzed. Structural and surface conditions, defects, and residual stress states play a crucial role in their operating performance in terms of light production, transport, and extraction. Industrial production of such crystalline materials is a complex process that requires sensing, in-line and off-line, for material characterization and process control to properly tune the production parameters. Indeed, the scintillators’ quality must be accurately assessed during their manufacture 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 the quality and structural condition of anisotropic scintillating crystals. Different inspection techniques (XRD, SEM, EDX, and 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 offers detailed structural and morphological information. Conoscopy and sphenoscopy offer the advantages of fast and non-invasive measurement suitable for the inspection of the whole crystal quality. These techniques, based on different measurement methods and models, provide different information that can be cross-correlated to obtain a complete characterization of the scintillating crystals. Inspection methods will be analyzed and compared to the present state of the art.
Highlights
Scintillating crystal-based detectors are the main devices used for radiation and particle detection.The scintillating substances, by converting radiation into visible light, produce the main signal, which is acquired by photoelectric sensors and analyzed by electronic and computing devices
Photoelasticity has been used for many decades for assessing the stress condition in isotropic and transparent materials like glass and Perspex; in some cases these materials are arranged in geometries similar to the mechanical and structural systems whose load-induced stress distribution have to be investigated [7,16,17]
Photoelasticity can be implemented in several modes; in all cases, photoelasticity generates fringe patterns that depend on the state of stress of the analyzed structure [16,17]
Summary
Scintillating crystal-based detectors are the main devices used for radiation and particle detection. Photoelasticity-based techniques [16,17] will be presented in the two different and complementary forms of laser conoscopy and sphenoscopy; these non-invasive techniques, and the associated theoretical models, allow us to collect information about residual stress, macro-defects and structural macro-distortion distribution, producing feedback for the tuning of the production parameters, ensuring the crystals’ correct light transport capability and predicting the functional behavior in a fast but reliable manner These methods are suitable for crystal producers, researchers, and end-users since they are non-destructive and all the samples of the production can be inspected. All these presented methods can be combined to arrive at a qualification procedure for the assessment of the crystals’ condition and the qualification of the production process
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