Accurate measurement of the first excited nuclear state in 235U

We have used superconducting high-resolution radiation detectors to measure the energy level of metastable ^Um₂₃₅ as 76.737 ± 0.018 eV. The ^Um₂₃₅ isomer is created from the α decay of ²³⁹Pu and embedded directly into the detector. When the ^Um₂₃₅ subsequently decays, the energy is fully contained within the detector and is independent of the decay mode or the chemical state of the uranium. The detector is calibrated using an energy comb from a pulsed UV laser. A comparable measurement of the metastable ^Thm₂₂₉ nucleus would enable a laser search for the exact transition energy in ²²⁹Th-^Thm₂₂₉ as a step towards developing the first ever nuclear (baryonic) clock.