Measurements in thorium-229 take a step towards the direct laser excitation of an atomic nucleus in this unique isotope — ScienceDaily


Nuclear clocks could make our time measurement even more accurate than atomic clocks. The key to this lies in thorium-229, an atomic nucleus whose lowest excited state has very low energy. A research team from the Kirchhoff Institute for Physics at the University of Heidelberg, TU Wien, Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), and GSI Helmholtzzentrum in Darmstadt has now succeeded in measuring this low energy. Using an extremely accurate detector, it was possible to detect the tiny temperature increase due to the energy released during the de-excitation of the atomic nucleus. This brings the realization of a nuclear clock a big step closer.

In radioactive decay, atomic nuclei spontaneously re-arrange, eject some part of their building blocks, and transform into a nucleus of a different atom. In this process, the new “daughter atom” usually has internally stored energy that is released in the form of gamma rays. The energies of these rays are characteristic for each type of nucleus — just like fingerprints. Researchers learn a lot about atomic nuclei by characterizing these gamma-ray fingerprints.

Back in 1976, L. A. Kroger and C. W. Reich investigated the decay of uranium-233, which is an artificial nucleus of uranium that decays to thorium-229 by emitting an alpha-particle; this is immediately followed by the emission of characteristic gamma-rays that occur in distinct and generally well-understood patterns. Kroger and Reich, however, registered an anomaly: one gamma-ray that was predicted by all nuclear theories was missing in the measured signals. The best explanation was that the internal energy stored in the lowest nuclear excitation of thorium-229 was too low to be observed by the detectors. Over the following decades, many attempts were made to observe this low-energy gamma-ray without success, constraining it to ever-lower energies.

New perspectives for constructing a