An excited state of Th 229 at 3.5 eV

It has been known for many years that the first excited state of Th229 lies close to the ground state. Originally this energy was given as <0.1 keV; later, the authors reported a value of -1±4 eV. In an attempt to improve the value for this level energy, we have remeasured the energies of a number of γ rays from U223 whose positions in the Th229 level scheme can be used to establish it. Compared with our earlier study, we have considered more γ rays in Th229, used more well-measured energy calibration and reference lines, used more detectors, used detectors with better low-energy resolution, more closely matched the counting rates in the γ-ray peaks whose relative energy is measured, and specifically considered certain systematic errors. More than 111 γ-ray spectra have been measured. From this large set of measurements we have deduced a value of 3.5±1.0 eV for the energy of this level.