Precise clocks underpin technologies ranging from satellite navigation to synchronized communication. Thorium-229 (229Th) is especially intriguing because it possesses an unusually low-energy metastable nuclear state that can be accessed with laser radiation at about 148 nanometers. Embedding 229Th in a transparent crystal such as calcium fluoride could enable compact solid-state devices with high dopant densities. However, interactions with the surrounding atoms can alter nuclear energy levels and influence the behavior of 229Th nuclei, and exactly how the crystal environment affects the nuclear transition remain unclear.
Addressing this challenge, a research team led by Specially Appointed Assistant Professor Takahiro Hiraki and Professor Koji Yoshimura from the Research Institute for Interdisciplinary Science, Okayama University, Japan, together with Professor Thorsten Schumm from the Faculty of Physics, TU Wien, Austria, investigated 229Th embedded in calcium fluoride single crystals. The researchers used a narrow-linewidth vacuum ultraviolet pulsed laser to perform laser Mössbauer spectroscopy, extending a technique traditionally used to probe nuclear environments into the optical range. A custom detector system suppressed background radioluminescence, allowing weak nuclear signals to be measured with high signal-to-noise ratios. Their findings were published in Volume 393, Issue 6813 of the journal Science on August 20, 2026.
The team studied three crystals with different 229Th concentrations and scanned the laser frequency around the nuclear excitation frequency. The resulting spectra contained numerous nuclear lines spanning three orders of magnitude in amplitude. By analyzing their splitting and relative intensities, the researchers showed that 229Th occupies four distinct microscopic sites in the calcium fluoride lattice. Each site experienced a distinct local electric field environment.
The two dominant sites together contributed more than 90% of the detected vacuum ultraviolet signal in all three crystals. Combining spectroscopy with density functional theory calculations, the researchers assigned microscopic models to the major configurations. Site 1, which showed a vanishing electric field gradient, was attributed to a charged thorium defect without local charge-compensating atoms. Site 2 was assigned to a cluster of two thorium atoms occupying nearest-neighbor calcium positions, also without local charge-compensating atoms.
"By resolving the nuclear spectra site by site, we established a way to investigate how 229Th atoms are incorporated into a solid and how their immediate surroundings affect the nuclear transition," says Prof. Hiraki. "This information is essential for understanding and engineering the crystal environments needed for future solid-state nuclear clocks."
Site-selective excitation also enabled the team to compare the isomeric-state lifetime and laser-induced quenching across the four environments. The radiative lifetime was approximately 630 seconds and showed no apparent dependence on the doping site, whereas quenching efficiency varied strongly between sites. Distinct local structures can change the electric field gradients experienced by thorium nuclei and split their energy levels into different spectroscopic patterns.
"The strong variation in quenching between the sites shows that the local crystal environment is not merely a passive host," says Prof. Yoshimura. "Identifying which structures preserve the nuclear excitation and which promote its loss will help us determine how to prepare crystals with improved performance."
The measurements therefore provide experimental information that can guide the selection, preparation, and optimization of materials for high-performance solid-state nuclear clocks.
Overall, the study establishes laser Mössbauer spectroscopy as a method for revealing the microscopic environments of 229Th in solids. By identifying four doping sites and characterizing how they influence the nuclear transition, the work supplies foundational data for designing compact nuclear clocks. More broadly, improved timing technologies could support advances in navigation, synchronized communication, and new forms of precision measurement, while the approach can be extended to other suitable host materials.
