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

TL;DR

This paper precisely measures the energy of the $^{229}$Th nuclear isomeric state using gamma-ray energy differences, confirming its accessibility for laser spectroscopy in the vacuum ultraviolet range.

Contribution

The study introduces a novel measurement of the $^{229}$Th isomer energy via absolute gamma-ray energy difference using a transition-edge sensor microcalorimeter.

Findings

Isomer energy determined to be 8.30±0.92 eV

Results agree with previous measurements, confirming laser-accessible range

Supports potential for high-resolution laser spectroscopy of the nucleus

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.