Search for ultraviolet and visible rays from $^{\mathrm{229m}}$Th

Ultraviolet- and/or visible-ray emission from the $^{\mathrm{229m}}$Th nucleus is indicated owing to its extremely low excitation energy of $\sim$ 7.4 eV. In addition, a drastic variation in the decay rate of $^{\mathrm{229m}}$Th depending on its chemical environment is also expected. Although many experimental results were reported for the observation of the deexcitation of $^{\mathrm{229m}}$Th, a consistent understanding of the deexcitation is considered not to be achieved. In this work, we separated $^{\mathrm{229m}}$Th from its mother nuclide $^{233}$U and prepared $^{\mathrm{229m}}$Th samples with several chemical forms such as chloride, nitrate, and hydroxide. Photon counting was performed for the samples with three types of photomultipliers for ultraviolet and visible rays (4--10, 1.9--7.2, or 1.4--6.8 eV). In the counting for 1.4--7.2 eV photons, no significant photon emission was observed for all the samples. In the presentation, we will show the results including those for higher energy photons.