The 1-hour neptunium activity which had previously been assigned to ${\mathrm{Np}}^{241}$ has been identified as the lower isomer of ${\mathrm{Np}}^{240}$. The decay energy of the 1-hour ${\mathrm{Np}}^{240}$ has been found to be 2.05 Mev compared with 2.18 Mev for that of the 7.3-min ${\mathrm{Np}}^{240}$. Gamma rays of energies 1160, 1000, 915, 595, 565, 435, 245, 160, and 85 kev have been found to be associated with the decay of ${\mathrm{Np}}^{240}$. The best value for the half-life of ${\mathrm{Np}}^{240}$ is 63±2 min. The isotope ${\mathrm{Np}}^{241}$ has been found to have a 16-min half-life with strong evidence for an isomer with a 3.4-hour half-life.

• Received 27 October 1959

DOI:https://doi.org/10.1103/PhysRev.118.263