Diffraction of Cathode Rays by Calcite

Abstract

A MONOCHROMATIC beam of cathode rays was directed against a cleavage face of calcite at a grazing incidence, and the diffraction pattern was obtained on the photographic plate placed behind the crystal normal to the incident beam. The energy of cathode rays, which were generated in a gas tube worked by an induction coil, was about 50 kilo electron-volts, the wave-length of the corresponding material waves being about 0.055 A. In the photograph reproduced (Fig. 1) is shown one of the patterns, which was obtained when the incident beam was perpendicular to [110] axis of the crystal and made an angle of 6° with the cleavage face. The photographic plate was placed 6.4 cm. away from the crystal. As will be seen, the pattern consists of a number of bands of different widths (for example, [111] in the figure), and also many black and white lines (e.g. [130] in the figure), ‘black’ and ‘white’ being referred to the negative. It resembles the pattern which is produced when the cathode rays are transmitted through a mica sheet of certain thickness (S. Kikuchi, Proc. Imp. Acad. Japan, 4, 271, 275, 354; 1928). Usually a black line makes a pair with a white line parallel to it. When the distance between the lines becomes small, the pair looks like a band. In fact, one edge of a band is bounded by a black line and the other edge by a white. There is no doubt that the band is nothing but a pair of lines separated by a short distance. Moreover, some of the bands show satellites which may be regarded as other pairs of black and white lines parallel to the main bands (for example, [131] in the figure, though difficult to recognise in the reproduction).