Microradiography Using Secondary Electrons*)

Using secondary electrons produced by x-rays incident on the specimen under study, it is possible to obtain remarkable images that reveal the structure of the object. Two methods of such imaging are distinguished: transmission of the object under study by secondary electrons, and recording the image produced by reflected secondary electrons. The resulting images, somewhat similar to microphotographs, are called microradiograms.

This method for studying the surface of an article or the structure of a sufficiently transparent object has existed for about 8 years. During this time, confidence has been gained in the expediency of its application in a very large number of cases.

Let us first consider microradiograms obtained with the aid of reflected secondary electrons. X-rays pass through black paper, through the film, and then fall on the object. The film is pressed with its emulsion against the object, and the success of the method is determined by the fact that the film is acted upon by photoelectrons reflected from the object, and not by x-rays that have passed through it. Thus, first of all it is necessary that the absorption coefficient of x-rays by the film be negligibly small and, conversely, that the film be very sensitive to electrons. The photographic layer must therefore be very thin and consist of very

) J. J. Trillat, J. Appl. Phys. 19*, 844 (1948).

small and insensitive grains. The X-rays must be hard; consequently, one must work at high voltages.

In special films used for this purpose, the thickness of the emulsion reached \(1/300\) mm. Emulsions with silver chloride may be used; they are weakly sensitive to X-rays and very sensitive to electrons. As for the voltage on the X-ray tube, which is satisfactory for these experiments in terms of hardness, it must lie between 100 and 200 kilovolts.

Of course, in the method described only the thin surface layer of the specimen is studied, since the electrons do not emerge from appreciable depths.

The success of this method is determined to a considerable degree by the fact that the electron emission increases with increasing atomic number of the substance. In the author's original paper radiograms are given that were obtained with the aid of electrons reflected from a flat surface consisting of strips of metals of different atomic numbers. It turns out that even metals neighboring one another in the periodic table, such as Fe and Ni, give clearly different blackenings of the film. Working in the range of voltages 150–200 kilovolts (here the effect of secondary characteristic rays is negligible in comparison with the action of photoelectrons), the author obtained excellent radiograms with exposures of a few seconds.

A number of applications of the method described may be indicated. First of all, this is the investigation of alloys and mixtures of metals. In view of the smallness of the grain of the photographic emulsion, the radiogram obtained can be greatly enlarged, and we see clearly the microstructure of a region measuring fractions of a square centimeter. In this way, a single photograph gives the microstructure of a large region of the surface.

The investigation of ores and minerals by this method is highly expedient. We distinctly see on microradiograms uranium grains and grains of uranium compounds in uranium ore. We distinguish, in the form of grains, PbS from NiAs\(_2\) and from SiO\(_2\). There is no doubt that this method is of great interest for geologists and mineralogists.

The excellent prints of radiograms presented in the author's work show that quite remarkable results can be achieved. The main condition is an almost ideal contact between the photographic layer and the specimen. Large absorption and diffusion of electrons in air, with loose contact, can completely spoil the microradiogram.

To test the mechanism by which the radiogram is produced, the author placed celluloid \(1/20\) mm thick in the path of the photoelectrons. The microradiogram did not appear. This proves that the images obtained are indeed the result of the action of photoelectrons, since both the characteristic X-rays of metals and the X-rays would pass through this obstacle without any difficulty.

The second of the methods developed by the author is the production of microradiograms by transillumination with secondary electrons. A sheet of black paper is placed in the path of the X-rays, then a sheet of lead 0.2 mm thick. This lead sheet serves as a source of photoelectrons, which pass through a very thin specimen (thousandths of a millimeter). Behind the specimen is the photographic film. Both the photoelectrons of the lead and the electrons knocked out of the specimen being transilluminated fall on the film. Under these conditions, at voltages of the same order as in the first method, i.e., 150–200 kilovolts, the number of photoelectrons proves to be quite sufficient, while the X-rays, both primary and secondary characteristic rays, do not leave an appreciable trace on the photographic film. Here again we use a fine-grained film and, upon enlargement, obtain a number of interesting details in a photograph of specimens measuring fractions of a square centimeter.

This method has been successfully applied for transilluminating paper, insect wings, animal and plant tissues. It is especially interesting to apply

Microradiography Using Secondary Electrons

...enlargement of these radiograms for specimens opaque to visible light (a microscope is of no use). The author applies this method to the study of thin films of lubricating substances that have been in contact with a part. It proves possible to decipher the structure of films up to 2 microns thick.

In order to determine the optimal conditions for electron emission, special investigations were carried out. It was shown that recoil electrons need not be taken into account—the entire effect belongs to photoelectrons. The distribution of the number of electrons by direction is very sharp. In the direction of the incident beam the emission is 1.2 times greater than in the opposite direction. The asymmetry increases as the wavelength and the atomic weight decrease. It is further shown that only atoms of the surface layer of lead participate in the emission (0.08 mm for \(L\)-electrons).

It should be noted that the laws of absorption of photoelectrons differ sharply from the law of absorption of X-rays. X-ray absorption depends approximately on the fourth power of the atomic number; electron absorption depends on density and has no simple relation to atomic number. For X-rays the dependence on atomic number is much sharper; thus, for example, X-rays are absorbed in gold 20 times more strongly than in aluminum, whereas for electrons the absorption coefficient in gold is only three times greater than the corresponding coefficient for aluminum. This must be kept in mind when interpreting microradiograms. Careful study of the nature of photoelectron absorption is of significance not only for interpreting microradiograms, but also for images observed in the electron microscope.

A. K.