Measurements of spontaneous fission of ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}_{}^{239}{}_{}{}^{}$ show that neutron irradiation of this substance forms a body with a high spontaneous-fission rate. This substance is with practical certainty ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}_{}^{240}{}_{}{}^{}$ and has a spontaneous-fission rate of approximately 1.6×${10}^6$ fissions/g hr. ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}_{}^{239}{}_{}{}^{}$ itself has a spontaneous-fission rate of the order of 40 fissions/g hr.

• Received 10 November 1953

DOI:https://doi.org/10.1103/PhysRev.94.156