A mass spectrographic determination of the relative abundances of the uranium isotopes has been made. The following abundance ratios were found: ${\mathrm{U}}^{238}$/${\mathrm{U}}^{235}$=139±1 percent; ${\mathrm{U}}^{238}$/${\mathrm{U}}^{234}$=17,000±10 percent. A search was made for other isotopes but none was found. From the above values, together with the results of the alpha-particle counting experiments of Kovarik and Adams, and the assumption that the actinium series is 4.6 percent as active as the uranium, the following decay constants are computed: ${\mathrm{U}}_{\mathrm{I}}$, 1.520×${10}^{-10\mathrm{}}$ ${\mathrm{yr}.\mathrm{}}^{-1\mathrm{}}$; ${\mathrm{U}}_{\mathrm{II}}$, 2.6×${10}^{-6\mathrm{}}$ ${\mathrm{yr}.\mathrm{}}^{-1\mathrm{}}$; AcU, 9.72×${10}^{-10\mathrm{}}$ ${\mathrm{yr}.\mathrm{}}^{-1\mathrm{}}$. The corresponding half-lives are, respectively, 4.56×${10}^9$ yr., 2.7×${10}^5$ yr. and 7.13×${10}^8$ yr.

• Received 3 November 1938

DOI:https://doi.org/10.1103/PhysRev.55.150