OBSERVATION OF THE SHADOW FROM AN ELECTRON BEAM

Smith and Hansen*) have described the following curious observation. Working with a cathode-ray tube, they illuminated a fluorescent screen with a broad beam of slow (50-volt) electrons, which experienced strong acceleration only in the immediate vicinity of the screen, in front of which there was a grid at a potential of 3000 volts. An auxiliary electron gun, located off the axis of the tube,

) H. Smith and S. Hansen, J. Appl. Phys., 23*, 1055 (1952).

created an additional electron beam, focused on the fluorescent screen. The field of the space charge of this beam deflected the slow electrons of the first beam that passed near the second; as a result, a “shadow” of the beam was formed on the screen, clearly visible in the photograph shown. (The position of the shadow is indicated by an arrow.) The bright spot in the upper part of the photograph is the trace of the focused beam (for this beam the exposure time during photography proved excessively long). The authors indicate that, in visual observation of the screen, the shadow is visible very distinctly, so that, for example, if the second beam is focused somewhat closer to the surface of the screen, then the shadow changes the place of its focusing along with it. The effect described may find practical application in studying the propagation of electron beams.

N. M.