X-Ray Microscopy

the aggregate of techniques used to investigate the microscopic structure of objects by means of X-radiation; it makes use of special devices—X-ray microscopes. The limit of resolution of X-ray microscopes may be two or three orders of magnitude better than that of optical microscopes, since the wavelength λ of X-radiation is two to three orders of magnitude shorter than the wavelength of visible light.

The specificity of the interaction of X rays with matter accounts for the difference between X-ray optical systems and optical systems for light waves and electrons. The small deviation of the refractive index of X rays from unity (less than 10⁴) in practice does not allow lenses and prisms to be used for focusing. Electronic and magnetic lenses are also unusable for this purpose, since X rays are inert with respect to electric and magnetic fields. Total external reflection of X rays by curved mirrors or reflection from crystallographic curved surfaces (reflection X-ray microscopy) is therefore used to focus X rays. By virtue of the high penetrating power of X rays, the simplicity of the line structure of the spectrum, and the strong dependence of the absorption coefficient of X-radiation on the atomic number of an element, X-ray microscopy can be carried out by the projection method in a divergent beam of rays emitted by a point source (projection X-ray microscopy).

The reflection X-ray microscope contains a microfocus source of X-radiation, curved mirror reflectors made of glass (gold-plated quartz) or curved single crystals, and image detectors (photographic plates or image converters). Figure 1 shows a diagram of the path of the rays in an X-ray microscope with two mirrors set at 90° with respect to each other. Achieving high resolution in a reflection X-ray microscope is limited by the small angle of total external reflection (the grazing angle is <0.5°) and, consequently, by the large focal distances (>1 m) and very rigid requirements on the quality of the finishing of the mirror surface (the permissible roughness is ~10 Å). The overall resolution of reflection X-ray microscopes is determined by the diffraction effect, which is dependent on λ, and by the angular aperture, which does not exceed the grazing angle. For example, for radiation with λ = 1 Å and a grazing angle of 25’, the diffraction-limited resolution does not exceed 85 A (magnification up to 100,000 diameters). Even when the mirrors are precisely figured, the images formed by reflection X-ray microscopes are distorted by the various aberrations of optical svstems. such as astigmatism and coma.

Figure 1. Diagram of the focusing of X rays in a reflection X-ray microscope with two crossed mirrors: (OO’) optical axis of the system, (A) object, (A’) image of the object. The magnification is O’A’/OA.

When curved monocrystals are used to focus the X-radiation, in addition to geometric distortions, the image quality is also affected by structural imperfections in the monocrystals and by the finite value of the Bragg diffraction angles (seeDIFFRACTION OF X RAYS).

Reflection X-ray microscopes have not been widely used because of technical difficulties in their manufacture and operation.

Projection X-ray microscopy is based on the principle of the shadow projection of an object in a divergent X-ray beam emitted by a point source (Figure 2). A projection X-ray microscope consists of a supermicrofocus X-ray source with a focus 0.1–1 micrometer (μm) in diameter (for example, a special microfocus X-ray tube, or a pinhole diaphragm combined with a conventional wide-focus X-ray tube) and a chamber that houses the object under study and the recording device.

The magnification M in projection X-ray microscopy is determined by the ratio of the distances from the point source of the X-radiation to the object (a) and to the detector (b): M

Figure 2. Diagram of a projection X-ray microscope using a wide-focus X-ray tube and a pinhole diaphragm

= b/a (see Figure 3). Consequently, the object must be located at a small distance from the point source. For this purpose the focus of the tube is located directly at the window of the X-ray tube or at the tip of the anode filament located near the tube window.

Figure 3. Formation of a penumbra R_g and diffraction fringe in a projection X-ray microscope

The linear resolution of projection X-ray microscopes reaches 0.1–0.5 μm. The geometric resolution is determined by the extent of the blurring (penumbra) R_g of the edge of the object, which depends on the size d of the X-ray source and on the magnification M: R_g = Md. The diffraction resolution depends on the Fresnel diffraction fringe at the edge of the object: R_d = αλ^½ where a is the distance from the source to the object. Since a in pratice cannot less than 1 μm, the resolution for λ = 1 Å will be 100 A, provided the dimensions of the source allow such a geometric resolution. Contrast in the image results from the differing absorption of X-radiation in regions of the object having different density or composition. The sensitivity of the projection X-ray microscopy technique is determined by the difference between the X-radiation absorption coefficients for different sections of the object under study.

Projection X-ray microscopy is widely used in medicine, mineralogy, metallography, and other fields of science and technology to investigate the microscopic structure of various objects. The X-ray microscope makes it possible to assess the quality of painting or thin coatings or of the gluing or finishing of miniature products. It makes possible microradiography of biological and botanical sections up to 200 μm thick. It is also used in the analysis of mixtures of powders of light and heavy metals and in the study of the internal structure of objects opaque to light rays and electrons. When this method is used, the specimens under study need not be placed in a vacuum, as with an electron microscope, and they are not subjected to the destructive action of electrons. Various devices for converting X-ray images into visible images in X-ray microscopes, combined with television systems, make it possible to achieve operational control of objects under scientific research and manufacturing conditions.