Visualization of X-ray beam using CdWO₄ crystal for macromolecular crystallography

In synchrotron diffraction experiments, it is typically assumed that the X-ray beam at the sample position is uniform, stable and has dimensions that are controlled by the focus and slits settings. As might be expected, this process is much more complex. We present here an investigation of the properties of a synchrotron X-ray beam at the sample position. The X-ray beam is visualized with a single crystal scintillator that converts X-ray photons into visible light photons, which can be imaged using Structure Biology Center (SBC) on-axis and off-axis microscope optics. The X-ray penetration is dependent on the composition of the scintillator (especially the effective Z), and X-ray energy. Several scintillators have been used to visualize X-ray beams. Here we compare CdWO₄, PbWO₄, Bi₄Ge₃O₁₂, Y₃Al₅O₁₂:Ce (YAG: Ce), and Gd₂O ₂S: Tb (phosphor). We determined that scintillator crystals made of CdWO₄ and similar high-Z materials are best suited for the energy range (7-20 keV) and are most suitable for beam visualization for macromolecular crystallography applications. These scintillators show excellent absorption, optical, and mechanical properties.