The low-energy excited state of the thorium-229 (229Th) isotope has recently attracted significant interest because it could serve as an excellent candidate for ultra-precise nuclear clocks. Building high-precision clocks necessitates a detailed understanding of the nuclear excitation and de-excitation processes. In this context, researchers have developed 229Th-doped vacuum ultraviolet (VUV) transparent CaF2 crystals. They utilized X-rays to manipulate the population of the isomeric state and monitored the radiative decay in a controlled chemical setting.
Atomic clocks allow scientists to measure time’s smallest standard unit—the second—with remarkable accuracy. These clocks rely on the natural oscillations of electrons within atoms, similar to the way pendulums function in traditional grandfather clocks. The pursuit of even more accurate timekeeping led to the innovation of nuclear clocks, which hinge on the transitions of atomic nuclei rather than electrons to track time.
A promising candidate in the race to create ultra-precise nuclear optical clocks is the first-excited state of the 229Th isotope. With a long half-life of 103 seconds and a low excitation energy of just a few electron volts, it is particularly suitable for excitation by VUV lasers, thus offering a precise reference transition for nuclear clocks. Additionally, nuclear clocks have potential applications in compact solid-state metrology devices and fundamental physics exploration. To tap into the possible uses of 229Th isomer, understanding its fundamental attributes—like isomeric energy, half-life, and the nuances of excitation and decay—is critical.
In pursuit of these insights, Assistant Professor Takahiro Hiraki from Okayama University in Japan, along with his colleagues Akihiro Yoshimi and Koji Yoshimura, devised an experimental setup to effectively evaluate the population of the 229Th isomeric state and observe its radiative decay. Their findings, recently published in Nature Communications on July 16, 2024, detailed the synthesis of 229Th-doped VUV transparent CaF2 crystals and showcased how they could manipulate the population of the 229Th isomeric state using X-rays. “Our team is focused on fundamental physics using atoms and lasers. Achieving a solid-state nuclear clock with 229Th relies on the ability to control the excitation and de-excitation states of the nucleus. In this study, we’ve successfully managed nuclear states using X-rays, moving us closer to realizing a nuclear clock,” explained Assistant Professor Hiraki regarding the motivation behind their research.
To explore the process of radiative decay (de-excitation), the team induced excitation from the ground state of the 229Th nucleus to an isomer state via the second excited state using a resonant X-ray beam. They discovered that the doped 229Th nucleus exhibited radiative decay back to the ground state, accompanied by the release of a VUV photon.
A notable discovery was the swift decay of the isomer state when subjected to X-ray beam irradiation, demonstrating the “X-ray quenching” effect, which enabled the selective reduction of the isomer population. The researchers are optimistic that this intentional quenching could facilitate advancements in nuclear clock technology, as well as other potential uses, such as portable gravity sensors and enhanced precision GPS systems.
Highlighting the promise of nuclear optical clocks, Assistant Professor Hiraki states, “Once we finalize the nuclear clock we are developing, it will allow us to investigate whether certain ‘physical constants,’ particularly the fine structure constant—which was once thought to be unchanging—might actually fluctuate over time. If we observe variations in the time of physical constants, it could shed light on dark energy, one of the universe’s greatest mysteries.”
