The diffraction of cathode rays by thin celluloid films

Abstract

In the de Broglie theory of mechanics a moving particle behaves as a group of waves of wave-length = h √¯1 — v 2 / c 2 / m 0 v and phase velocity V = c 2 / v , where m 0 is the mass of the particle, v its speed and c the velocity of light. In the case of electrons of energy 10,000 to 40,000 volts, the corresponding wave­ lengths would be from 1·22 X 10 -9 to 0·66 X 10 -9 cms. These waves, there­ fore, should behave in many ways like hard X-rays, and, in particular, should show similar diffraction patterns when passed through crystals. Thus, accord­ing to the Bragg formula, if the rays are incident at an angle θ on a set of parallel planes, we should get reflection provided that 2 d sin θ = nλ,d being the spacing between the planes. To investigate these effects, a series of experiments, suggested and super­vised by Prof. G. P. Thomson, was begun in October, 1926. An account of these has already been published in ‘ Nature,’ but since then much more accurate work has been done. The experiments consisted in sending a beam of homogeneous cathode rays through a thin film at normal incidence and receiving them on a photographic plate. As shown below, the resultant pattern should be of the Hull-Debye-Scherrer type. Celluloid was chosen because it is comparatively easy to get films of the order of 500 Ă. U. thick. These were pre­pared by dissolving celluloid in amyl acetate. A little of the solution was dropped on water and the amyl allowed to evaporate. The celluloid which remained was removed on cardboard frames. It is essential that the films be thin enough to prevent blurring of the pattern by multiple scattering.