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UBC Theses and Dissertations

A cloud chamber study of pair production Wolfe, Harry Bernard


The present status of the Bethe-Heitler theory of pair production is analysed, and results are given which can be compared with experiment. The main points of interest in the pair formation process are the cross-section, energy and angular distribution of the electrons, the momentum imparted to the nucleus, and the manner in which these factors vary with photon energy and atomic number. Although the assumptions involved, such as the Born Approximation, appear to be justified, a review of the literature shows that experimental results have not always been in entire agreement with theory. For instance, the experimental distribution of Ē₊-Ē₋ follows neither the Bethe-Heitler nor the Jaeger-Hulme theory. The proposed experiment is to be carried out with a cloud chamber using Xenon as the gas, and ThC" as the γ-ray source. The errors involved in the method are discussed. To minimize scattering error a new method of analysis of cloud chamber tracks is suggested, in which the angle between successive equidistant chords is measured. The nine inch chamber is of the rubber diaphragm type. The operation of the chamber and camera has been made entirely automatic. The magnetic field is obtained by a pair of Helmholz coils. Two General Electric F.T. 126 flash lamps provide sufficient light for photography. Stereoscopic pictures are obtained by the double mirror method. A great number of difficulties had to be overcome, especially in the functioning of the expansion valves, in order to get good electron tracks and consistent operation. It has been found that it is very important to use correct procedure in filling the chamber and "cleaning" it out for the production of tracks. Very nice electron tracks have been obtained with a ThC" source and Argon in the chamber. Preliminary observations indicate that the source may need to be shuttered, and that the chamber will need a thin window.

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