ANGLE CALCULATIONS FOR A 6-CIRCLE SURFACE X-RAY DIFFRACTOMETER

Angle calculations are presented for a new type of diffractometer suit able for surface X-ray diffraction. The new geometry results from comb ining the four-circle and the z-axis geometries and involves six circl es. Instruments based on this concept can be operated in different dif fraction geometries. Compared to the four-circle and z-axis geometries , a larger fraction of the reciprocal space perpendicular to the sampl e is available to experiments when all six circles are used. This resu lts in an improved out-of-plane resolution in any X-ray structure stud y. In each geometry, either the angle of incidence or the outgoing ang le of the X-rays can be chosen. At the same time, the surface normal c an be aligned in the horizontal diffractometer plane, providing optimu m resolution and sample illumination. All equations are given in close d form. The different modes of operation are compared and operation sc hemes are discussed.