A new isotope of the element emanation has been identified among the spallation products of thorium bombarded with 110-Mev protons. This isotope, ${\mathrm{Em}}^{221}$, decays with a half-life of 25 minutes. The decay goes 80 percent by beta emission to ${\mathrm{Fr}}^{221}$ identified by its characteristic alpha particle groups and by the observance of the alpha particles of the decay products, ${\mathrm{At}}^{217}$ and ${\mathrm{Po}}^{213}$. In 20 percent of its disintegrations ${\mathrm{Em}}^{221}$ emits an alpha particle of 6.0-Mev energy unresolved from the 6.0-Mev alpha particle of ${\mathrm{Fr}}^{221}$, and gives rise to ${\mathrm{Po}}^{217}$, a previously unreported isotope of polonium. ${\mathrm{Po}}^{217}$ has a half-life of less than 10 seconds and emits alpha particles of 6.54±0.02 Mev.

An attempt was made to isolate ${\mathrm{Em}}^{221}$ from a sample of ${\mathrm{Th}}^{229}$ where it should be found as a result of slight alpha branching of ${\mathrm{Ra}}^{225}$. The experiment was negative and a lower limit of 500 years was placed on the partial alpha half-life of ${\mathrm{Ra}}^{225}$.

• Received 29 August 1955

DOI:https://doi.org/10.1103/PhysRev.101.136