Laser spectroscopic characterization of the nuclear clock isomer $^{229m}$Th

TL;DR

This paper reports laser spectroscopic measurements of the $^{229m}$Th nuclear isomer, providing key nuclear properties and advancing the development of a highly precise nuclear clock based on this unique low-energy nuclear state.

Contribution

It presents the first detailed laser spectroscopic characterization of the $^{229m}$Th isomer, including nuclear moments and charge radius, enabling non-destructive optical detection methods.

Findings

Measured magnetic dipole and electric quadrupole moments.

Determined the nuclear charge radius of $^{229m}$Th.

Established a method for optical detection of the isomer.

Abstract

The isotope $^{229}$Th is the only nucleus known to possess an excited state $^{229m}$Th in the energy range of a few electron volts, a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than common nuclear excitation energies. A number of applications of this unique nuclear system, which is accessible by optical methods, have been proposed. Most promising among them appears a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of $^{229m}$Th$^{2+}$, yielding values of fundamental nuclear properties, namely the magnetic dipole and electric quadrupole moments as well as the nuclear charge radius. After the recent direct detection of this long-searched-for isomer, our results now provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.