Ultraviolet Photon Emission Observed in the Search for the Decay of the229ThIsomer

Ultraviolet spectroscopy of ${}^{233}\mathrm{U}$ and ${}^{232}\mathrm{U}$, in the form of uranium oxide dissolved in 3M nitric acid, has revealed a number of spectral lines with widths of less than 6 nm from the ${}^{233}\mathrm{U}$ source but no detectable narrow lines from the ${}^{232}\mathrm{U}$ source. These lines could possibly be attributed to the decay of the ${}^{229}\mathrm{Th}$ isomeric state at an energy of around 4.0 eV.