Isotopic Constitution of Uranium

A. J. Dempster

Editor’s Note

Uranium has the highest atomic weight of any naturally occurring element, and exists predominantly in the form of uranium-238. Here, however, physicist Arthur Dempster of the University of Chicago reports that about 0.4% of the element is an unstable isotope uranium-235. This isotope, he suggests, is the origin of the actinium series of radioactive elements, as the elements protactinium-231, actinium-227, francium-223 and so on can be generated by radioactive alpha and beta decays. Dempster also notes that another isotope, uranium-234, appears to exist in traces of 0.008%. Uranium-235 would soon become very important as the only natural fissile isotope: one capable of sustaining a nuclear chain reaction driven by slow moving neutrons, thus making nuclear power and atomic bombs conceivable.ft　　中文

THE analysis of uranium rays from the volatile hexafluoride by Dr. Aston1 has shown a single line at atomic weight 238. The element appeared to be simple to at least two or three percent, but its properties were not favourable for study in the gas discharge. As uranium is of great importance for the subject of radioactivity, the spark source described in Nature of April 6 (135, 542) was tried with uranium metal and gold as electrodes, and also with an electrode made by packing a nickel tube with pitchblende. It was found that an exposure of a few seconds was sufficient for the main component at 238 reported by Dr. Aston; but in addition on long exposures a faint companion of atomic weight 235 was also present. With two different uranium electrodes it was observed on eight photographs, and two photographs with the pitchblende electrode also showed the new component. The relative intensity could be only roughly estimated on account of the irregularity of the spark, but it appeared to be less than one percent of the intensity of the main component.ft　　中文

This faint isotope of uranium is of special interest as it is in all probability the parent of the actinium series of radioactive elements. In discussing Dr. Aston’s analysis2 of the isotopes in lead from radioactive minerals, Lord Rutherford3 pointed out that the lead isotope of atomic weight 207 is probably the end product of the actinium series, so that the atomic weight of protoactinium would be 231, (207+6×4). This value has been verified by the recent chemical determination of the atomic weight by v. Grosse4. Protoactinium itself may be formed by α- and β-ray transformations from a hypothetical isotope of uranium, actino-uranium, with an atomic weight of 235 or 2393. The relative amount of actino-uranium at present on the earth would be 0.4 percent of the uranium according to a recalculation by Dr. v. Grosse5,6. The present observations thus support this theory, with the atomic weight of 235 for the isotope actino-uranium. A third isotope, uranium II, of atomic weight 234 amounts theoretically to only 0.008 percent of the uranium, and would be too faint for observation by the mass-spectrograph.ft　　中文

(136, 180; 1935)

A. J. Dempster: University of Chicago, July 12.

References:

  1. Aston, Nature, 128, 725; 1931.

  2. Aston, Nature, 123, 313; 1929.

  3. Rutherford, Nature, 123, 313; 1929.

  4. A. v. Grosse, Proc. Roy. Soc., 150, 363; 1935.

  5. A. v. Grosse, Phys, Rev., 42, 565; 1932.

  6. A. v. Grosse, J. Phys. Chem., 38, 487; 1933.