Coincidence studies of radioactive isotopes in the ${\mathrm{Ac}}^{225}$ decay chain were made. A metastable state of ${\mathrm{Pb}}^{209}$ with a half-life of (3.1±1.0) millimicroseconds was observed. This delay following beta decay of ${\mathrm{Tl}}^{209}$ to ${\mathrm{Pb}}^{209}$ is exhibited by a 120-kev $E1\mathrm{}$ gamma transition and other gamma transitions succeeding the 120-kev gamma transition. Upper limits are set for the lifetimes of several other gamma transitions present in the ${\mathrm{Ac}}^{225}$ chain. An explanation for the delayed nature of this 120-kev $E1\mathrm{}$ transition is given in terms of parentage overlap. Some unusual features of the beta decay rates of ${\mathrm{Tl}}^{209}$ to ${\mathrm{Pb}}^{209}$ are discussed.

• Received 17 April 1956

DOI:https://doi.org/10.1103/PhysRev.103.748