The absence of any detectable ${\mathrm{Pu}}^{243}$ daughter activity in a sample of curium containing two micrograms of ${\mathrm{Cm}}^{247}$ indicates that the alpha half-life of ${\mathrm{Cm}}^{247}$ exceeds 4×${10}^7$ years and probably exceeds 9×${10}^7$ years. The possibilities of finding ${\mathrm{Cm}}^{247}$ in nature and of finding high ${\mathrm{U}}^{235}$: ${\mathrm{U}}^{238}$ ratios in very old rare-earth minerals are discussed.

• Received 1 October 1956

DOI:https://doi.org/10.1103/PhysRev.105.679