This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, physicists have used a technique called Mössbauer spectroscopy to peer inside solid materials, revealing fine details of their surroundings by studying how their atomic nuclei absorb and re-emit gamma rays. Through new research published in Science, a team led by Takahiro Hiraki at Okayama University has pushed this approach into entirely new territory, using visible-adjacent light instead of gamma rays to study the nuclei of thorium-229 atoms.
The advance offers a promising new route toward nuclear clocks, devices that could one day keep time more precisely than anything built before. Within a solid crystal, atomic nuclei don't sit in perfectly uniform surroundings. Since neighboring atoms produce minute electric fields, nuclear energy levels shift slightly in a way that reveals the local structure around each nucleus.
These shifts can be measured using a technique called Mössbauer spectroscopy, now a valuable tool for measuring solid materials in chemistry and geology. Nuclear clocks, meanwhile, have some way to go before they can be practically implemented. To date, atomic clocks are the most accurate timekeepers available, measuring time by tracking the frequencies of energy-level transitions taking place in atomic electrons.
In contrast, nuclear clocks can track transitions taking place deep inside the nucleus, making them far less sensitive to stray electric and magnetic fields. For now, the gamma-ray lasers needed to trigger these transitions aren't yet available—with one key exception. In the isotope thorium-229, the nuclear transition sits at an unusually low energy, reachable with an ultraviolet laser rather than a gamma-ray source.
In their study, Hiraki's team combined this quirk with Mössbauer-style analysis, doping thorium-229 into calcium fluoride crystals and probing it with a narrow, tunable vacuum ultraviolet laser. Their results revealed that thorium ions can settle into four distinct positions within the crystal lattice, each leaving a characteristic fingerprint in the surrounding electric field. By tuning their laser to address each site individually, the researchers could selectively excite thorium nuclei at a chosen location.
Then, they could measure how long that excited state lasted before decaying and how laser light itself could speed up that decay. Not all four sites are equally useful, however. For a practical nuclear clock, thorium atoms need to sit in a uniform, predictable environment.
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