The projection X-ray microscope and related microanalytical techniques

Abstract

Three methods of microscopical investigation with X-rays are in process of active development: the total reflection, the contact microradiographic and the direct projection methods. The total reflection method, in which true focussing of X-rays takes place at polished cylindrical mirrors, has been investigated very thoroughly by Kirkpatrick and Baez (1) and a detailed account of its present state has been given by Kirkpatrick and Pattee (2). It has attained a resolving power of about 0.25 μ, but further progress is hindered by technical difficulties of preparing the specially curved surfaces required to reduce the severe aberrations that occur in conditions of glancing incidence. The contact method is the simplest, since the specimen is photographed at one-to-one magnification on a very fine grained emulsion, without the need for any optical element in either the electron or the X-ray path. It has been developed to a high degree of usefulness in biological research by Engström (3, 4) in particular, who has also investigated its value as a method of microanalysis by differential absorption of X-rays [see also Lindström (5), Clemmons (6)]. As the micro-negative must be enlarged through an optical system, in order to make available the stored information, the resolving power is limited to that of the best optical microscope and this limit (about 0.2 μ) has now been reached. Efforts have been made (7, 8) to circumvent this difficulty by using electron microscopy to enlarge the photographic image, but the difficulties are formidable. The projection method of X-ray microscopy, on the other hand, has no such limitation because it provides a direct initial magnification of the image, which is recorded on a normal type of emulsion and requires little or no subsequent enlargement. A resolving power of about 0.1 μ has already been obtained (9) and further improvement is possible. The projection method is closely related to electron microscopy, since electron lenses are employed to obtain the very small focal spot by which the image is projected. The resolution and exposure time depend on the size and intensity of the spot, and therefore on the aberrations of the electron lenses. For this reason it is appropriate to discuss the physical aspects of projection X-ray microscopy in the present meeting. Attention will be directed mainly to advances made since the 1956 Symposium on X-ray microscopy (10).