Unsymmetrical and symmetrical -phi assemblies on a four-circle X-ray diffractometer

Abstract

This paper discusses the relative advantages of the unsymmetrical and symmetrical χ-phi assemblies as they are used in four-circle X-ray diffractometers. Ignoring any limitations due to other parts of the apparatus, such as collimator or detector, the blind region in the half of the limiting sphere corresponding to hkl reflections occupies a volume which is less than ¼% of the volume of the hemisphere for the unsymmetrical assembly and 4% for the symmetrical assembly. In the other half of the limiting sphere, corresponding to hkl reflections, the figures are 13% and 20%, respectively, for the two assemblies. The symmetrical assembly has no blind region in the usable part of the hkl hemisphere and can always be used with the plane of the χ-circle bisecting the angle between the incident and reflected beams. The unsymmetrical assembly is easier to construct with accuracy and reliability than the symmetrical assembly.