Findings from the experimental setup indicate the feasibility of a small, portable nuclear clock.

Scientists use atomic clocks to measure the smallest unit of time, the second, with great precision. Atomic clocks use the natural oscillations of electrons in atoms, similar to how the pendulum of an old grandfather clock works. In search of even more precise timekeeping devices, atomic clocks were discovered that use transitions in atomic nuclei to measure time.

The first nuclear excited state of the 229Th isotope is a promising candidate for the development of an ultra-high precision nuclear optical clock. With a long half-life of 103 seconds and low excitation energy of a few electron volts, it is ideal for excitation by a vacuum ultraviolet (VUV) laser and provides an accurate reference transition for the nuclear clock.

Nuclear clocks can also be used in small solid-state instrumentation and fundamental physics research. To explore potential applications of 229Th isomers, a detailed understanding of their fundamental properties, such as isomer energies, half-lives, and excitation and decay dynamics, is essential.

Continuing research in this direction, a team led by Assistant Professor Takahiro Hiraki, Akihiro Yoshimi, and Koji Yoshimura of Okayama University has developed an experimental setup to effectively assess the population of the 229Th isomeric state and detect its radioactive decay.

In a study published in Nature Communications on July 16, 2024, the team synthesized VUV-transparent CaF2 crystals doped with 229Th and demonstrated that they could use X-rays to control the population of the 229Th isomer state.

“In our laboratory, we conduct research into fundamental physics using atoms and lasers. To realize a solid-state nuclear clock using 229Th, we need to control the excited and de-excited states of atomic nuclei. In this research, we have succeeded in controlling the state of atomic nuclei using X-rays, bringing us one step closer to realizing a nuclear clock,” said Assistant Professor Hiraki, explaining the motivation for their research.

To investigate radiative decay (de-excitation), the team used a resonant X-ray beam to generate excitations of 229Th nuclei from the ground state through a second excited state to an isomeric state. They found that the doped 229Th nuclei undergo radiative decay to the ground state with the emission of a VUV photon.

One key discovery is the “X-ray quenching” effect, whereby isomeric states decay rapidly when exposed to X-ray beam irradiation, allowing the density of isomers to be reduced on demand. The researchers believe that this controlled quenching could facilitate the development of nuclear clocks, along with other potential applications such as portable gravity sensors and high-precision GPS systems.

Assistant Professor Hiraki emphasizes the potential of nuclear optical clocks, saying, “Once the nuclear clock we are developing is completed, we will be able to verify whether the ‘physical constants’ that were previously thought to be unchanging, in particular the fine structure constant, change over time. If we can observe the change in physical constants over time, this may lead to the elucidation of dark energy, one of the greatest mysteries of the universe.”