We have used superconducting high-resolution radiation detectors to measure the energy level of metastable 235mU as 76.737 ± 0.018 eV. The 235mU isomer is created from the α decay of 239Pu and embedded directly into the detector. When the 235mU 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 229mTh nucleus would enable a laser search for the exact transition energy in 229Th−229mTh as a step towards developing the first ever nuclear (baryonic) clock.
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