A Wilson Cloud Chamber Investigation of the Alpha-Particles from Uranium

The mean ranges of the alpha-particles emitted by uranium have been determined in terms of the mean range of polonium alpha-particles. The ratios obtained for the mean ranges are, for U I/Po, 0.6904\ifmmode\pm\else\textpm\fi{}0.0007, and for U II/Po, 0.8357\ifmmode\pm\else\textpm\fi{}0.0008. These correspond to mean air ranges of 2.63 and 3.18 cm, for the particles from U I and U II, respectively, in dry air at normal pressure and at 15\ifmmode^\circ\else\textdegree\fi{}C, on the basis of the present polonium value of 3.80 cm. The straggling coefficient was determined for the polonium alpha-particles used as the reference range group in the experiment and gave a value considerably closer to theoretical prediction than the usual experimental results. Possible evidence for the existence of a new alpha-emitting isotope of uranium is suggested in the data. The mean air range for the new group is believed to be in the vicinity of 2.9 cm. An identification with the actino-uranium isotope of approximately this range suggested by Wilkins as the parent of the actinium series and with Dempster's 235 uranium isotope is proposed.