X-ray microscopy and microanalysis

The development of X-radiography for medical purposes was one of the early successes in the application of X-rays to practical problems, and the attractions of applying radiography on the microscopic scale, for study of small biological specimens or sections, are obvious. In recent years, methods of viewing small objects by means of a magnifying system using X-rays have been developed and are in use in many biological and metallurgical laboratories.

The first exploratory studies appear to have been made by Sievert (1936). In this work an aperture a few micrometres in diameter was placed in front of an X-ray tube, enabling magnified images to be produced by shadow projection. A system such as this would suffer from the rather small intensity of X-radiation which would be available through a small aperture used in this way, but photographs with a resolution of 5–10 μm were obtained.

However, developments in electron optics were necessary before projection X-ray microscopy, at anything approaching optical resolution, became a practical proposition. Von Ardenne (1939) proposed the use of an electron lens for demagnifying an electron source, thereby enabling X-rays to be generated in a region with a diameter of the order of a few micrometres only. An X-ray tube using this principle was constructed by Cosslett and Nixon (1952), and the method of projection X-ray microscopy has undergone continuous development from that time onwards, and has become an investigational method of considerable importance in biology and metallurgy.