X-RAY MICROSCOPE*)

It is usually considered that focusing X-rays by means of lenses or mirrors is impossible. Roentgen himself was convinced of this, and for several decades after him physicists shared his opinion with great disappointment. The realization of an X-ray microscope would open up a whole field of research inaccessible to the light microscope because of its insufficient resolving power, and to the electron microscope because of the low penetrating power of electron beams.

Although all bodies do refract X-rays, the effect is so weak that it is detected only with difficulty. An ordinary eyeglass lens has a focal length for X-rays of the order of 100 km. A number of substances have a larger refractive index, but, unfortunately, all such substances at the same time absorb X-rays very strongly. Apparently, it may be assumed that substances which are at once transparent to X-rays and strongly refracting do not exist.

The possibility of reflective optical systems, at first sight, is likewise not very favorable. Reflection from crystal surfaces is sufficiently weak, and the chromatic aberrations are large. Moreover, it is hardly likely that the structure of the surfaces of real crystals is sufficiently regular for the formation of images.

) L. P. Kirkpatrick, Nature 166*, 251 (1950).

There remains the possibility of using total reflection—an effect independent of the crystalline nature of the reflecting body. This total reflection differs from the total internal reflection of visible light in that, in the case of X-rays, it is easier to obtain total external reflection, which is undoubtedly much more convenient. On the other hand, total reflection of X-rays is possible only at angles of incidence close to a right angle, i.e., under the condition of “glancing,” tangential incidence of the beam on the reflecting surface. The glancing angle is then usually considerably less than one degree. Despite such unfavorable conditions, the formation of images by concave mirrors is nevertheless possible. If a small radiation source \(O\) is placed before a concave mirror, as shown in Fig. 1, the reflected rays, obeying the ordinary laws of reflection, collect at the point \(F\). In this case the mirror may possess considerable converging power with respect to rays diverging from the source in the plane of the drawing; with respect to rays diverging perpendicular to the drawing, the focusing action is extremely small.

Fig. 1.

Naturally, an image from a single mirror is astigmatic to the highest degree, i.e., a point object gives an image in the form of a line. Kirkpatrick proposed and implemented a method for eliminating astigmatism, consisting in the use of crossed mirrors (Fig. 2). The result of the action of such an optical system is entirely analogous to the correction of astigmatism of the eye by eyeglass lenses with additional astigmatism (the so-called cylindrical lenses). As is seen from the figure, the rays emerging from the point, being successively reflected in two mirrors, again collect into a point.

Fig. 2.

On the basis of similar crossed reflecting systems, with the source and object in the appropriate positions, the first experiments in X-ray microscopy were carried out. In Fig. 3 is shown the magnified image obtained by Kirkpatrick of a frequent

Fig. 3.

of a metallic mesh placed between the target of the X-ray tube and the first mirror of the pair, like that shown in Fig. 2. The direct magnification, somewhat different in perpendicular directions, was of the order of 50; the photograph was additionally enlarged photographically by a factor of 10. The small spheres near the middle of the figure represent images of metal droplets on the mesh, produced electrolytically; their diameter reaches approximately 1 micron.

Since the theoretical focal surface of the system is inclined to the reflected rays, the image on the photographic film, normal to them, is focused only at one point—in Fig. 3 close to one of the metal spheres. The accuracy with which the edge of this sphere can be located from the photograph may be estimated as one wavelength of the visible part of the spectrum. Of course, from the photograph in Fig. 3 it is by no means possible to draw any new conclusions about the object under consideration; the photograph is given as an example of what has already been achieved at the present time.

The mirrors of the “X-ray microscope” are made by ordinary methods from glass and then coated with a thin layer of a heavy metal by evaporation in vacuum. The metallic coating makes it possible to work at large critical glancing angles. The mirror surfaces used were spherical, with radii of curvature of 10–100 m. In this case it is not necessary that both mirrors of the pair have the same surface curvature.

Undoubtedly, the use of aspherical surfaces, for example mirrors of elliptical profile, will make it possible to improve the quality of the images considerably. Elliptical mirrors will be able to eliminate spherical aberration; however, the inconvenience of an inclined focal surface and unequal magnification in different directions will remain. It is possible that more complex optical systems, for example two pairs of mirrors, will prove necessary. Unfortunately, the question of image formation in the grazing incidence of beams on a surface apparently has not been treated at all in theoretical optics. It is possible that X-ray microscopy in particular will draw attention to this area of theory.

The short wavelength of X-rays, in principle, makes it possible to obtain high resolving power; however, as in the electron microscope, the angular apertures of the beams must be very small.

In the case of X-rays the angular aperture is sharply limited by the value of the critical angle for total reflection; increasing it by increasing the wavelength does not lead to an increase in resolving power.

Theoretically, in the limiting case, resolution is possible for point objects at a distance of the order of 70 angstroms from one another.

From the standpoint of convenience of operation it is desirable not to use excessively hard rays, since this increases the critical angle, which makes it possible to reduce the exposure. In the case of working with thin organic objects, soft radiation gives greater contrast. In Kirkpatrick’s experiments the wavelength was usually 2–3 Å. In view of the strong absorption of such rays by air, the optical system was placed in a vessel filled with helium at a pressure of 1 atm. Focusing was carried out by changing the inclination of the mirrors with a micrometric device.

Reflecting systems are free from chromatic aberration; therefore, for image formation there is no need for a monochromatic source. However, in the case of total reflection the mirrors themselves constitute a kind of selector, reflecting the long-wavelength and absorbing the short-wavelength part of the incident radiation, which leads to an improvement in image contrast.

It is quite possible that X-ray “microscopes” will soon become a valuable instrument in a number of fields of research. In particular, the possibility of studying biological objects that have not been subjected to the desiccating action of a vacuum and to electron bombardment, which in a number of cases interfere with work with an electron microscope, is very interesting. On the other hand, there is the possibility of a wide range of applications in metallography. The selectivity of X-ray absorption can probably be used for chemical analysis of specimens when they are studied under an X-ray microscope in monochromatic rays.

V. Vavilov