# Laser Cooling of Radium Ions

## TL;DR

This paper demonstrates laser cooling of radium ions in a tabletop setup, enabling precise measurements of atomic states and transition frequencies despite the challenges posed by radium's radioactivity and lack of stable isotopes.

## Contribution

The authors successfully laser-cooled radium ions in a low-activity source and measured key atomic properties, advancing experimental techniques for radioactive ion research.

## Key findings

- Measured branching fractions of the 7p state to ground and metastable states.
- Determined the 7s to 7p transition frequency with high precision.
- Demonstrated laser cooling of radium ions in a low-activity experimental setup.

## Abstract

The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity ($\sim 10 \ \mu\mathrm{Ci}$) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the $7p\ ^2P_{1/2}^o$ state's branching fractions to the ground state, $7s\ ^2S_{1/2}$, and a metastable excited state, $6d\ ^2D_{3/2}$, to be $p=0.9104(7)$ and $0.0896(7)$, respectively. With a nearby tellurium reference line we measured the $7s\ ^2S_{1/2} \rightarrow 7p\ ^2P_{1/2}^o$ transition frequency, 640.09663(6) THz.