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Slow-motion spectroscopy paves the way for a nuclear clock

Slow-motion spectroscopy paves the way for a nuclear clock

12 April 2024

For the first time, the energy level of an atomic nucleus has been directly manipulated with a laser.

Illustration of thorium-229 atoms inside a calcium fluoride crystal.
Thorium-229 atoms, shown in light blue in this artist’s impression, are held in the lattice of a calcium fluoride crystal. The isotope is prized for its unusually low-lying nuclear excited state—just 8.338 eV above the ground state, a small enough energy gap to be bridged with a laser. Credit: Oliver Diekmann, Technical University of Vienna

The discrete colors of light absorbed and emitted by atoms have long been integral to our understanding of the physical world. During a solar eclipse in 1868, an unexpected yellow line in the Sun’s spectrum led to the discovery of a new element, helium. A few decades later, atomic energy levels, deduced from their spectra, helped drive the development of quantum mechanics. And atomic clocks, the best of which waver by less than a second over the age of the universe, are the basis for GPS and the definitions of the fundamental units of measure themselves (see the article by David Newell, Physics Today, July 2014, page 35).

Atomic nuclei also have discrete energy levels, but there’s no nuclear optical spectroscopy, and there aren’t yet any nuclear clocks. The trouble is that almost all nuclear transitions lie at impractically high energies: in the realm of gamma rays, not visible light. The closest thing to an exception is a vacuum-ultraviolet (VUV) transition in the rare radioactive isotope thorium-229. But until recently, researchers didn’t know its energy with anywhere near the precision they’d need to look for it in a spectrum. And the excited state’s long radiative lifetime, more than 10 minutes, makes it hard to tell whether the nucleus has been excited or not.

Now a team of researchers led by Thorsten Schumm (Technical University of Vienna) and Ekkehard Peik (National Metrology Institute of Germany) has succeeded in exciting 229Th with a laser. The achievement builds on three key results published last year. Researchers at CERN detected photons emitted by 229Th nuclei (which had been created in the excited state by radioactive decay of other elements) and pinned down the transition energy to 8.338 ± 0.024 eV, or a wavelength of just over 148 nm. Peik and colleagues designed and built a tunable laser system at that wavelength. And Schumm and colleagues developed a way to trap large numbers of 229Th atoms in a perfect, VUV-transparent calcium fluoride crystal.

Observing the nuclear excitation was still a painstaking process. A fraction of a picometer at a time, the researchers scanned their laser wavelength over the target range, just a fraction of which is shown in the figure below. At each stage, they left the laser on for two minutes, then switched it off for two minutes to collect any fluorescence photons that the 229Th nuclei emitted. But because that cycle isn’t long enough to allow all the excited nuclei to return to the ground state, the apparent position of the spectral peak shifted, depending on whether the wavelength was scanned from low to high or from high to low.

A graph showing the peak emission wavelength from the thorium-229 nucleus.
Laser excitation of the thorium-229 nucleus looks different from different directions. It takes many minutes for the nuclei to completely relax back to their ground state, so the apparent position of the peak depends on which way the laser wavelength is scanned. Credit: Adapted from J. Tiedau et al., Phys. Rev. Lett. (in press)

The researchers were motivated by the goal of using the 229Th transition as the basis for an ultraprecise clock. They have much more work left to do to achieve that dream. But if and when it’s realized, a nuclear clock could, among other things, improve the precision of GPS signals and measurements of Earth’s gravitational field. Researchers could then better track subtle movements of the ground around volcanoes and seismic faults, which could help them forecast eruptions and earthquakes. (J. Tiedau et al., Phys. Rev. Lett., in press.)

Editor’s note, 19 April: The article has been updated to include the uncertainty on the 229Th transition energy that was obtained in the 2023 CERN study.

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