Speaker
Description
Among thousands of nuclei, the isotope Thorium-229 ($^{229}$Th) is the only nucleus with an excitation level (isomer state) of about 8 eV and has attracted attention as a nucleus that can be excited by a laser. Nuclei are less sensitive to external fields than atoms and can achieve extremely stable quantum states. One promising application is the “nuclear clock”, which could potentially outperform the atomic clock. The isomeric transition could also be utilized for many fields, including fundamental physics and various practical applications.
To excite $^{229}$Th by lasers, detailed information on isomer-level energies, lifetimes, and other related details is needed. In 2016, the existence of the isomer state was confirmed by observing the internal conversion electrons [1]. Since then, many experiments have reported the detailed information on the isomer state [2-5]. In 2023, the first radiative decay from the isomer was observed and its precise energy was reported [6]. It is expected that research will rapidly progress toward the first laser excitation.
Aiming at further study of isomer state in crystals, we have been developing a new method using nuclear resonant scattering with synchrotron radiation X-ray. This "active X-ray pumping" allows us to produce isomer states from the ground state $^{229}$Th in a controllable manner [7]. By utilizing this method to $^{229}$Th-doped CaF$_2$ crystals, we have successfully observed the deexcitation vacuum ultraviolet (VUV) radiation from the isomer. This result leads to the detailed study of isomers in CaF$_2$ crystals.
In this talk, I will discuss our VUV observation. In addition, I will also give an overview
of our XAFS(X-ray Absorption Fine Structure) experiments to probe the electronic state of Th in crystals, which provide important information for manipulating the isomer in the crystals.
[1] L.v.d. Wense et al., “Direct detection of the $^{229}$Th nuclear clock transition”, Nature ,533, 47 (2016)
[2] B. Seiferle et al., “Energy of the $^{229}$Th nuclear clock transition”, Nature, 573, 243 (2019).
[3] J. Thielking et al., “Laser spectroscopic characterization of the nuclear-clock isomer $^{229m}$Th”, Nature 556, 321 (2018).
[4] A. Yamaguchi et al., Energy of the 229Th Nuclear Clock Isomer Determined by Absolute $\gamma$-ray Energy Difference”, Phys. Rev. Lett. 123, 222501
[5] T. Sikorsky et al., “Measurement of the $^{229}$Th Isomer Energy with a Magnetic Microcalorimeter”, Phys. Rev. Lett. 125, 142503 (2020)
[6] S. Kraemer et al., “Observation of the radiative decay of the $^{229}$Th nuclear clock isomer”,Nature 617, 706 (2023)
[7] T. Masuda et al., “X-ray pumping of the Th-229 nuclear clock isomer”, Nature, 573, 238 (2019).
