Search for decay of the 3.5-eV level in ²²⁹Th

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