X-ray beam position monitors

This invention relates to X-ray beam position monitors. An X-ray beam position monitor has a first plate assembly () for detecting the position of the X-ray beam in a horizontal plane and a second plate assembly () for detecting the position of the X-ray beam in a vertical plane. The first plate assembly () and the second plate assembly () are located at the same position along the direction of propagation of the X-ray beam, to provide a compact arrangement suitable for use with X-ray diffractors and for laboratory use.