The geometric selection and optical system optimization of the angle measuring instrument in powder diffractometers are the core technologies for improving the quality of diffraction data, and their design needs to take into account focusing efficiency, resolution, and ease of operation.
The geometric selection of the goniometer is dominated by Bragg Brentano (BB) geometry, which achieves quasi focusing conditions by rotating a planar specimen and a detector with a 2:1 angular velocity ratio. The radius of the focal circle of this geometry varies with the diffraction angle, with the center point of the sample strictly located on the focal circle, while there is defocusing in the edge region. However, by controlling the incident light divergence (such as using a variable slit), high intensity can be maintained at the diffraction peak position. For complex shaped samples (such as gear tooth roots), the tilt method rotates the sample around a horizontal axis to avoid absorption effects on diffraction geometry and improve low angle diffraction accuracy, especially for residual stress measurement.
The optimization of the optical system focuses on upgrading the optical path module. Traditional BB geometry relies on diverging slits (DS) and receiving slits (RS) to control beam divergence, while modern instruments introduce Soller Slits, which restrict vertical divergence through parallel metal foil arrays and control axial divergence angles within 2.26 °, significantly reducing defocusing effects. Parallel light path systems (such as G ö bel mirrors) convert divergent X-rays into parallel beams through multi-layer film reflection, eliminating K β radiation and white light interference, and improving diffraction peak resolution. For example, the TRIO optical path system of the Brooke D8Discover diffractometer can automatically switch between BB geometry, parallel light geometry, and high-resolution monochromatic optical path, adapting to the diverse testing needs of powders, thin films, and single crystal epitaxial films.
The synergistic optimization of target material and detector further eliminates fluorescence interference. For samples containing copper and nickel, the BBHD module can filter out continuous white light and K β radiation; For iron, cobalt, and manganese elements, a 1Der full wavelength energy dispersive detector eliminates fluorescence background with an energy resolution of 340eV. For example, the cobalt target BBHD module combined with a 1Der detector can clearly identify weak diffraction peaks of cementite Fe3C in steel samples, breaking the detection limit of traditional optical paths.