Some investigations on the scattering and absorption of heterogeneous beams of X-radiation

Abstract

The variability of experimental results frequently obtained by different workers in this laboratory from experiments conducted under apparently well controlled conditions, caused investigations to be carried out under Professor Barkla's direction to try to determine what influence was producing the variations. The idea was expressed that the results could not be explained by, current theories and that a hitherto unknown physical process was at work. Ideas regarding the nature of tnis unknown quantity changed with time as new results were obtained, but all efforts to reach a satisfactory position were un- successful. When it became impossible to explain the J- discontinuities in terms of a characteristic radiation, the hypothesis was put forward that heterogeneous radiation behaves differently from the sum of its homogeneous components, especially as regards the scattering process. According to this theory, a beam of heterogeneous radiation has properties depending on some average quality of the beam, which Professor Barkla suggested to be something equivalent to a 'temperature of the radiation'. Since a close connection between scattering and filtering experiments could be established, it was only natural to assume that the governing factor would be the same in both experiments.Such was the situation when this work was begun.The main object of the researches carried out in this laboratory in the past few years was to throw some light on that quantity the 'temperature of the radiation', a problem that could be attacked from several sides. The problems with which the writer was faced were therefore to investigate (1) the reason why apparatus A and B in one room gave different results forthe scattering experiment (s/P against thickness of filtering aluminium at constant KV) than apparatus C and D in another room. (2) the nature of the sudden change in the slope of the graphs which is so characteristic of most results of the scattering experiment (S /P against KV.) (3) the unknown controlling factor which causes the J-discontinuity to appear and disappear without any apparent reason and which changes the result of the scattering experiment from one type into another one.In addition to this, another problem arose in connection with the scattering experiment, namely the fact that in some experiments a pinhole primary aperture had been found to give a result which is entirely different from the result obtained with relatively larger primary apertures. This problem did not seem to have any connection with the problems mentioned before, as it could be strictly controlled just by inserting one or other of the primary apertures.An investigation was conducted by the writer into the influence of the apertures on the result of the scattering experiment. It was found that the result (i.e. the slope of the graph) depends only on the sizes of the incident apertures J' and J and not, within the limits of normal working, on the apertures subsequent to the scatterer limiting the primary and secondary beams. These experiments were not continued, once it had been established that the difference in the results could not be ascribed to an unknown fundamental physical quantity.In the scattering experiment, the investigations of the sudden change in slope of the graph did not lead to positive results, for, with the exception of a few early experiments, these abrupt changes could not be reproduced despite extensive changes in experimental conditions: changes which according to other workers would have changed the results completely. Although the writer was not successful in solving this particular problem, an important fact emerged from the work. Provided the experimental conditions were varied within the normal working limits, the results were all similar and certainly not contrary to theory. This statement covers variations in the scatterer material and thickness, in apertures subsequent to the scatterer, in the X-ray tube current and covers also the use of 3 different X-ray tubes.As regards the filtering experiment, it was impossible to observe any trace of a J-discontinuity.As in the scattering experiments, the results were remarkably uniform. In the filtering experiments observations have, for the first time, been made free from errors due to polarization of the primary beam.The experimental work which has been reported in this thesis has given results which,within the limits of accuracy of observation, are not in conflict with accepted theory. No evidence has been obtained of a J-discontinuity and none of any discontinuous process in the scattering experiment.