Although most people are probably familiar with the different energy levels that the electron shells of atoms can be in and how electrons shedding excess energy as they return to a lower state emit for example photons, the protons and neutrons in atomic nuclei can also occupy an excited state. This nuclear isomer (metastable) state is a big part of radioactive decay chains, but can also be induced externally. The trick lies in hitting the right excitation wavelength and being able to detect the nuclear transition, something which researchers at the Technical University of Wien have now demonstrated for thorium-229.

The findings by [J.Tiedau] and colleagues were published in Physical Review Letters, describing the use of a vacuum-ultraviolet (VUV) laser setup to excite Th-229 into an isomer state. This isotope was chosen for its low-energy isomeric state, with the atoms embedded in a CaF₂ crystal lattice. By trying out various laser wavelengths and scanning for the signature of the decay event they eventually detected the signal, which raises the possibility of using this method for applications like new generations of much more precise atomic clocks. It also provides useful insights into nuclear isomers as it pertains to tantalizing applications like high-density energy storage.

Although the difference in the amount of energy between the stable and semistable isomer state of a nucleus is minor in absolute terms, when compared to chemical and other forms of energy storage it can be significantly larger. Storing and releasing this energy has been the subject of research going back decades, with a 2008 paper by [E. P. Hartouni] at Lawrence Livermore National Laboratory on ^178m2Hf in particular concluding that it was ‘highly improbable’ to become a practical form of energy, but this has not kept research on the topic from progressing.

One consideration here is that the number of nuclear isomers is massive, and their properties are quite distinct, as noted in a 2024 review paper by [Bhoomika Maheshwari] and colleague, along with the realization that we still miss a lot of fundamental understanding on the topic of these nuclear states. In a 2021 research paper by [Yuanbin Wu] and colleagues the long-term energy storage potential and controlled release of energy in a ^93mMo isomer is studied, using electron beams as the trigger. Although still early days, this kind of research may be the path to many new technologies related to time-keeping, computing and energy storage.