Search for decay of the 3.5-eV level in ${}^{229}\mathrm{Th}$

We used a novel radiochemical technique to search for the decay of the 3.5-eV level ${(}^{229}{\mathrm{Th}}^m)\mathrm{in}{}^{229}\mathrm{Th}$ to the ground state ${(}^{229}{\mathrm{Th}}^g)$ by separating “fresh” ${}^{229}\mathrm{Th}$ from approximately 25 g of ${}^{233}\mathrm{U}.$ We then measured its γ-ray decay as a function of time seeking evidence of ${}^{229}{\mathrm{Th}}^g$ growth from ${}^{229}{\mathrm{Th}}^m$ isomeric decay. No evidence of such growth was observed. From this experiment we have concluded that the ${}^{229}{\mathrm{Th}}^m$ half-life must be either <6 h or >20 days.