The spontaneous-fission half-life for the odd-odd nucleus ${\mathrm{Am}}^{242m}$ has been measured by a prompt-neutron fission-fragment coincidence method to be (9.5±3.5)×${10}^{11}$ years. A correlation of experimental spontaneous-fission half-life data for nuclei of all classes was found to be ${log}_{10}\tau -(\beta -\frac{Z^2}{A})\delta M=-9.268\left(\frac{Z^2}{A}\right)+374.0\mathrm{}$, where $\beta$ takes the values 42.22, 40.67, and 40.40 for even-even, odd-$A$, and odd-odd nuclides, respectively, $\tau$ is the spontaneous-fission half-life in years, and $\delta M$ is the mass difference in milli-mass-units (mmu) between the values obtained from Green's smoothly varying mass formula and the experimentally measured values.

• Received 19 September 1966

DOI:https://doi.org/10.1103/PhysRev.155.1309