An exact knowledge of the mechanical and optical properties of crystals allows not only for theoretical advances, but it is also a useful tool to asses crystal quality in the technological processes of growth and production of advanced crystals like, eg, scintillators. In this paper, we study the elasto‐optic behavior of tetragonal crystals to evaluate the photoelastic constants, associated to various states of stress, in terms of the components of the piezo‐optic tensor. Moreover, we arrive at a generalization, for tetragonal crystals, of the Brewster law for optically isotropic materials.

Scintillator crystals like the tetragonal PbWO 4 (PWO) are largely used in calorimeters, for instance the CMS at CERN and in the PANDA calorimeter project. The performances of those calorimeters are directly linked to crystals homogeneity and quality, which can be assessed using indicators such as residual stress state condition. Since it is fundamental to develop and refine new mathematical models for the structural characterization and stress analysis of the crystalline materials, in this paper we present a novel approach aimed at the evaluation of stress or defect distribution and inhomogeneity, by analysing images obtained with photoelastic conoscopic system. A new model is proposed for the correct mathematical interpretation of the fringe pattern obtained by investigating the crystal in direction orthogonal to the uniaxial optical axis and parallel to the crystallographic a-axis. The model will be validated against experimentally acquired fringe patterns. A comparison with a simplified procedure, which operatively, reduces the computational effort, will be detailed. After a complete description of the fringe pattern, a linearized model is proposed in such a way that we obtain a simple stress intensity evaluation via experimentally easy measurable parameters, thus realizing a model based measurement of stress state. The results make the proposed procedure reliable, and useful also for other crystals symmetries. K: Detection of defects; Scintillators and scintillating fibres and light guides; Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators) 1Corresponding author.

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.

No abstract

The Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, provides unique possibilities for a new generation of hadron-, nuclear- and atomic physics experiments. The future antiProton ANnihilations at DArmstadt (PANDA or $$\overline{\mathrm{P}}$$ P ¯ ANDA) experiment at FAIR will offer a broad physics programme, covering different aspects of the strong interaction. Understanding the latter in the non-perturbative regime remains one of the greatest challenges in contemporary physics. The antiproton–nucleon interaction studied with PANDA provides crucial tests in this area. Furthermore, the high-intensity, low-energy domain of PANDA allows for searches for physics beyond the Standard Model, e.g. through high precision symmetry tests. This paper takes into account a staged approach for the detector setup and for the delivered luminosity from the accelerator. The available detector setup at the time of the delivery of the first antiproton beams in the HESR storage ring is referred to as the Phase One setup. The physics programme that is achievable during Phase One is outlined in this paper.