IMAGE ACQUISITION WITH ION BEAMS

As is known, electric and magnetic fields of the proper form can serve as lenses for electron beams. With such lenses it is possible to obtain sharp images of objects emitting electrons (electron optics). It is obvious that similar methods can also be applied to obtain images by means of ion beams. In this case, however, one must abandon the use of magnetic lenses, since in them the trajectory of a particle of a given charge depends on its mass; therefore, in passing from electrons to ions, in order to obtain identical trajectories it would be necessary to greatly reduce the velocity of motion of the particles, which for many reasons is impossible (the glow of the fluorescent screen), or else to change considerably the magnetic-field strength in the coils that create the magnetic field of the lenses. On the other hand, the use of lenses with an electrostatic field in the case of ion beams is quite possible, since in them, for particles of a given energy, the trajectory does not depend on the mass and is determined only by the magnitude of the charge.

Koch and Walcher* have recently succeeded in using singly charged potassium ions to obtain images of objects emitting these ions or illuminated by them. This became possible only after Koch succeeded in obtaining a sufficiently strong source—

* Koch u. Walcher, Z. Physik, 97, 131, 1935.

ion source giving a current of more than \(10^{-5}\) A from \(1\ \text{cm}^2\) of surface for many hours (more detailed data on the ion source are not yet available).

In the authors’ experiments the lens for ions was a cylinder cut into three parts by planes perpendicular to its axis; the outer parts were connected to the edges, and the same potential, different from the potential of the middle part of the cylinder, was applied to the outer parts (similar lenses had previously been used by Knoll in electron optics). To obtain images with the aid of ions, the authors used two such lenses placed one after the other; in this arrangement the first lens, placed in front of the anode—the ion source—collected ions on the “imaged” object, for example on a screen with a row of holes, while the second, placed on the other side of the “imaged” object, collected the ions that had passed through it on a fluorescent screen (after leaving the second lens the ions moved in a field-free space). By selecting the focal distances of the lens, depending on the potentials of the individual parts, an image of the “imaged” object was obtained on the screen and was then photographed in the usual way. Control experiments on deflecting the image in a magnetic field showed that the image is indeed obtained from positively charged ions.

With the aid of two lenses, images were obtained of screens with 2 and 25 holes. Although the images obtained are very blurred, they nevertheless make it possible to judge the outlines of the object being photographed. If one takes into account the fact that these are the first photographs taken with ion beams, they may for the time being be considered more or less satisfactory.

Photographs of the same objects in electron beams, taken with the ion source replaced by an incandescent tungsten cathode, show that the ion images give a greater magnification than the electron ones. This is explained by the fact that in the former case the effect of space charges, which distort the lens field and cause expansion of the ion beam, is manifested to a greater extent.

To obtain an image of the anode itself emitting ions, the authors placed one lens directly in front of it and, with its aid, obtained an image of the source on a fluorescent screen. In this way it is possible to trace the operation of the anode over different periods of its burning. Although in this case too the images obtained are not very sharp, nevertheless this method may find application in solving a number of questions connected with the emission of ions by surfaces.

L. Groshev, Moscow