Excitation and probing of low-energy nuclear states at high-energy storage rings

TL;DR

This paper explores methods to excite and probe low-energy nuclear states in $^{229}$Th ions at high-energy storage rings, proposing innovative laser techniques and schemes for high-precision energy measurements relevant for nuclear clocks.

Contribution

It introduces new laser excitation and probing schemes for $^{229}$Th nuclear states using relativistic ions, including electric dipole transitions enabled by nuclear hyperfine mixing.

Findings

High excitation rates achievable with relativistic $^{229}$Th ions.

Proposed electric dipole transitions for efficient isomer excitation.

Schemes for high-precision energy determination of nuclear isomers.

Abstract

$^{229}$Th with a low-lying nuclear isomeric state is an essential candidate for a nuclear clock as well as many other applications. Laser excitation of the isomeric state has been a long-standing goal. With relativistic $^{229}$Th ions in storage rings, high-power lasers with wavelengths in the visible range or longer can be used to achieve high excitation rates of $^{229}$Th isomers. This can be realized through direct resonant excitation, or excitation via an intermediate nuclear or electronic state, facilitated by the tunability of both the laser-beam and ion-bunch parameters. Unique opportunities are offered by highly charged $^{229}$Th ions due to the nuclear-state mixing. The significantly reduced isomeric-state lifetime corresponds to a much higher excitation rate for direct resonant excitation. Importantly, we propose electric dipole transitions changing both the electronic and nuclear states that are opened by the nuclear hyperfine mixing. We suggest using them for efficient isomer excitation in Li-like $^{229}$Th ions, via stimulated Raman adiabatic passage or single-laser excitation. We also propose schemes for probing the isomers, utilizing nuclear radiative decay or laser spectroscopy on electronic transitions, through which the isomeric-state energy can be determined with an orders-of-magnitude higher precision than the current value. The schemes proposed here for $^{229}$Th could also be adapted to low-energy nuclear states in other nuclei, such as $^{229}$Pa.