FROM THE CURRENT LITERATURE

ULTRASONIC MICROSCOPE

Recently, in the Reports of the Academy of Sciences, a communication was announced by S. Ya. Sokolov on the ultrasonic microscope invented by him[^1]. The operating principle of this microscope was proposed by S. Ya. Sokolov as early as 1936 and consists in the following (see Fig. 1):

The object under examination, 2, is “illuminated” by a narrow beam of ultrasonic rays produced by a piezoelectric quartz plate, 1.

Fig. 1.

The rays reflected from the object are collected by an acoustic lens, 3, at whose focus there is mounted a receiver—a piezoelectric quartz plate, 4. The ultrasonic rays cause deformation of this plate, as a result of which piezoelectric charges arise on its inner surface. The distribution of the charges corresponds strictly to the form of the ultrasonic field incident on the plate. Since the form of the ultrasonic field at the focus of the lens is determined by the contours of the object under examination, the distribution of the charges corresponds to the contours of the object: on the inner surface of plate 4 an “electric image” is obtained of the object, which must be made visible and at the same time magnified. Plate 4 is the bottom of a cathode tube, 5. A narrow beam of cathode rays, emitted by cathode 6, falls on its inner surface and knocks out secondary electrons, collected by anode 7. The piezoelectric charges forming the “electric image” of the object alter the secondary electron emission. These changes are amplified by a special device, 8, and transmitted to the modulation device of another

of the cathode-ray tube 9, where the intensity of the cathode beam is varied in accordance with the change in the secondary emission or, ultimately, with the contours of the object under examination. If, as is done in television, the cathode rays of both tubes are made to move synchronously along lines and frames, then the object under examination² will be visible on the screen of tube 9.

The magnification factor of the image is equal to the ratio of the linear dimensions of the beam frames of the two cathode-ray tubes. The calculations carried out show that an ultrasonic microscope is capable of providing thousandfold magnifications.

Fig. 2.

The sharpness of the image will be the greater, the smaller the cross-sectional area of the cathode-ray beam in tube 5 and the ratio of the ultrasonic wavelength to the dimensions of the object under examination. The author points out that, according to recent work, the frequency of ultrasonic waves can reach \(3 \cdot 10^9\) cycles. In this case the wavelengths are comparable with the wavelengths of visible light.

The advantage of the ultrasonic microscope over the optical one consists in the fact that with its aid one can obtain enlarged images of objects and inhomogeneities located not only in optically transparent media but also in opaque media. For example, Fig. 2 shows an image obtained in an ultrasonic microscope of a metal loop immersed in transformer oil, at tenfold magnification.

V. Leshkovtsev

CITED LITERATURE

1. S. Ya. Sokolov, DAN SSSR, LXIV, No. 3, 333 (1949); ZhTF, 19, issue 2, 271 (1949).
2. S. Ya. Sokolov, ZhTF 11, issue 1—2, 160 (1941).