A Cloud Chamber Study of the Compton Effect

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Abstract
An experiment is described which is designed to test the photon theory of scattering in the energy region between 0.5 and 2.6 MEV. A small pencil of radiation from a thorium source is shot through a cloud chamber. A celluloid scatterer 0.8 mm thick is placed in the beam at the center of the chamber. Two thin lead sheets are placed parallel to, and on each side of, the primary beam, to absorb the scattered photons. A magnetic field is applied to determine the energies of the electrons. From the energy and direction of an electron ejected from the scatterer, the direction of the scattered photon expected from theory can be calculated, and is independent of any assumption about the energy of the primary photon. Out of 10,000 photographs taken, 300 electron-photon combinations were found. The angular and energy relations of these seem to indicate that the photon theory of scattering is valid. It is shown experimentally that the scattering of the electrons in the celluloid is of the right order of magnitude to account, at least in part, for the observed deviations of the results from theory.

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