Energy of the ${}^{229}$Th Nuclear Clock Isomer Determined by Absolute $\gamma$-ray Energy Difference

Abstract

The low-lying isomeric state of ${}^{229}$Th provides unique opportunities for high-resolution laser spectroscopy of the atomic nucleus. We determine the energy of this isomeric state by taking the absolute energy difference between the excitation energy required to populate the 29.2-keV state from the ground-state and the energy emitted in its decay to the isomeric excited state. A transition-edge sensor microcalorimeter was used to measure the absolute energy of the 29.2-keV $\gamma$-ray. Together with the cross-band transition energy (29.2 keV$\to$ground) and the branching ratio of the 29.2-keV state measured in a recent study, the isomer energy was determined to be 8.30$\pm$0.92 eV. Our result is in agreement with latest measurements based on different experimental techniques, which further confirms that the isomeric state of ${}^{229}$Th is in the laser-accessible vacuum ultraviolet range.

6 pages, 4 figures, 2 tables

References in corpus (2)

• Direct detection of the 229Th nuclear clock transition
• Results of a direct search using synchrotron radiation for the low-energy ${}^{229}$Th nuclear isomeric transition

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