Decay Constant for Spontaneous Fission ofU238

Abstract

Uranium impurities in minerals undergo spontaneous fission over geologic times, leaving radiation-damage trails whose number can be related to the mineral age, provided ${\lambda }_F$, the spontaneous-fission decay constant of ${\mathrm{U}}^{238}$, is known. By requiring that the ages of a large number of minerals measured in this way agree with ages determined by decay of ${\mathrm{K}}^{40}$ and ${\mathrm{Rb}}^{87}$, a value ${\lambda }_F=6.9\mathrm{}\times {10}^{-17\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$ was deduced. By placing a sheet of natural uranium next to a sheet of mica for 6 months and then counting tracks entering the mica from the uranium, a value ${\lambda }_F=6.6\mathrm{}\times {10}^{-17\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$, in good agreement with the above value, was obtained. A weighted average of these two results gives the value ${\lambda }_F=(6.85\mathrm{}\pm 0.20)\times {10}^{-17\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$.