Search for the decay of^229mTh by photon detection

A search for the isomeric transition of Th-229m at 3.5 eV was made by photon detection with a low-noise photomultiplier for samples prepared by a rapid chemical separation from the parent nuclide U-233 by means of an anion exchange method. The photon emission caused by the disintegration of Th-229m was not observed for Th-229 nitric acid solution samples. From the experimental results, the limits of the half-life of Th-229m as dissolved in HNO3 solution are discussed.