Characterization of a defective PbWO₄crystal cut along the a-c crystallographic plane: structural assessment and a novel photoelastic stress analysis

Abstract

Among scintillators, the PWO is one of the most widely used, for instance in CMS calorimeter at CERN and PANDA project. Crystallographic structure and chemical composition as well as residual stress condition, are indicators of homogeneity and good quality of the crystal. In this paper, structural characterization of a defective PbWO 4 (PWO) crystal has been performed by X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Photoelasticity in the unusual (a, c) crystallographic plane. XRD and EDS analysis have been used to investigate crystallographic orientation and chemical composition, while stress distribution, which indicates macroscopic inhomogeneities and defects, has been obtained by photoelastic approaches, in Conoscopic and Sphenoscopic configuration. Since the sample is cut along the (a, c) crystallographic plane, a new method is proposed for the interpretation of the fringe pattern. The structural analysis has detected odds from the nominal lattice dimension, which can be attributed to the strong presence of Pb and W. A strong inhomogeneity over the crystal sample has been revealed by the photoelastic inspection. The results give reliability to the proposed procedure which is exploitable in crystals with other structures.

Quality Control and Structural Assessment of Anisotropic Scintillating Crystals

“…The inset on the right side shows the conoscopic images carried out over four points, while the larger image represents the sphenoscopic fringe pattern obtained in the same area by a single acquisition [14]. (b) The map of stress distribution superimposed to the sample surface, which is in line with the presence of defects in the PWO crystal [14] (used with permission from IOP; [14]). [14].…”

“…By observing orthogonally to the optic axis (or along the obtuse bisector of biaxial crystals), the interferogram appears as a series of hyperbolas (Figure 2a,b). In this case, the distance between the hyperbolas of the branches is linked to the stress condition by the following relation (Equation 10), which is derived from a more complex model [14]:…”

“…During conoscopy measurements, to obtain a symmetric fringe pattern (as in Figure 2), the sample was tilted to the same angular value (4 • ) from the observation direction. A fruitful combination of techniques can be found in a recent paper [14] on modern PWOs (faster and with enhanced light yield). In that work laser conoscopy and sphenoscopy were used to inspect a defective PWO crystal as a pre-series sample for the PANDA experiment in FAIR [9], [41] [9]; the assessment of the quality was completed by XRD and EDS analysis, obtaining structural and compositional information.…”

“…Although anisotropic crystals offer very high performance, they are very sensitive to stress, structural conditions, and defects [13,14]. Small changes in the production parameters or environmental conditions can create unwanted defects that dramatically affect the scintillating behavior, reducing the crystals performance and lifetime.…”

Influence of a Surface Finishing Method on Light Collection Behaviour of PWO Scintillator Crystals

“…In order to face these challenges, the light performances of scintillator materials need to be improved through theoretical efforts and experimental assessment [8,9]. The condition of scintillators must be accurately evaluated, and it is mandatory to finely tune the production process parameters via feedback from the crystal final state [10][11][12]. Structural conditions and elasto-optical properties of scintillators play a crucial role that must be deeply investigated [13,14].…”