A photographic investigation of the transmutation of lithium and boron by protons and of lithium by ions of the heavy isotope of hydrogen

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In the preceding paper, p. 722, Oliphant, Kinsey and Rutherford have given an account of the examination of the disintegration of lithium by protons and by ions of the heavy isotope of hydrogen, using electrical counting methods, and have shown that the results of their experiments lend strong support to the views of the modes of disintegration which they there suggest. Certain of these conclusions may be more completely examined by photographing the tracks of the disintegration particles in an expansion chamber and with that object we have made the experiments described below. In the course of this work a great amount of experimental data has been collected which will require a more detailed analysis; in the present paper the photographs described have been selected with the object of testing the above-mentioned theories. It is possible that the photographs show evidence of other modes of disintegration, but in view of the time required for a full analysis, we publish here only an account of the more obvious phenomena. The apparatus used for the production of the high voltage and its application to the tubes used for accelerating the bombarding particles was that described by Cockcroft and Walton, and potentials up to about 400 kilovolts were used. The first attempts to work an expansion chamber in conjunction with that apparatus showed that the maximum number of disintegrations produced per second was much too small, and considerable time was spent in attempting to obtain a more intense beam of protons. The form of discharge tube finally adopted was that described by Oliphant and Rutherford, This has been found much more definite in behaviour than the glass discharge tube used in the early work of Cockcroft and Walton. The total positive ion current measured at the target is at least ten times greater than could be obtained from the latter tube, and the number of disintegrations produced per second has been increased by an even larger factor. It is probable that the new type of tube gives a larger ratio of protons to molecular ions in the beam—this ratio being more liable to fluctuation with the glass discharge tubes. With the present arrangement, using the maximum output of current and voltage, it is possible to obtain over 100 tracks per expansion from a lithium target a few square millimetres in area.