The nucleus of thorium 229 has a metastable "isomer state" in the energy region of only a few eV (electronvolt) from the ground state.It is said to be the only isomer state that can be created by laser irradiation, and is attracting attention due to its potential application to ultra-high-precision "nuclear clocks."However, the previously reported energy values ​​of the isomer state up to 2007 did not match among the experimental groups, and new measurements were awaited.

　This research group has independently developed a gamma-ray spectroscope with high energy resolution called a superconducting transition edge sensor, and succeeded in accurately measuring the transition energy from the second excited state of the thorium 229 nucleus to the isomer state.Then, by taking the difference from the energy of the second excited state measured in the previous research, the energy of the isomer state was determined to be 2 ± 2 eV.This value was also consistent with the latest measurements by two other experimental groups, measured by different experimental methods in 8.30.

　In the future, it is expected that research will proceed toward the realization of precise laser spectroscopy of thorium 229 nuclei by producing an excitation laser in the energy region determined in this research.Atomic clocks are clocks made based on the frequency of electromagnetic waves absorbed when the atomic nucleus transitions from the ground state to the isomer state, and are said to be able to achieve an accuracy that is more than an order of magnitude higher than the current atomic clocks.It can be expected to be applied to various fields such as verification of homeostasis of basic physical constants, which have been pointed out to change with the expansion of the universe, and as a measurement tool for relativity geodesy.

Paper information:[Physical Review Letters] Energy of the 229Th Nuclear Clock Isomer Determined by Absolute γ-ray Energy Difference