# Catching, trapping and in-situ-identification of thorium ions inside Coulomb crystals of $^{40}$Ca$^+$ ions

## TL;DR

This paper demonstrates trapping and in-situ identification of thorium ions within Coulomb crystals of calcium ions, enabling non-destructive mass and charge analysis crucial for nuclear physics and quantum spectroscopy research.

## Contribution

It introduces a novel method for trapping and identifying thorium ions inside Coulomb crystals using calcium ions, combining optical imaging and time-of-flight verification.

## Key findings

- Successful trapping of single thorium ions in Coulomb crystals.
- Development of a non-destructive identification method for thorium ions.
- Verification of ion identification through time-of-flight measurements.

## Abstract

Thorium ions exhibit unique nuclear properties with high relevance for testing symmetries of nature, and Paul traps feature an ideal experimental platform for performing high precision quantum logic spectroscopy. Loading of stable or long-lived isotopes is well-established and relies on ionization from an atomic beam. A different approach allows trapping short-lived isotopes available as alpha-decay daughters, which recoil from a thin sample of the precursor nuclide. A prominent example is the short-lived $^{229\text{m}}$Th, populated in a decay of long-lived $^{233}$U. Here, ions are provided by an external source and are decelerated to be available for trapping. Such setups offer the option to trap various isotopes and charge states of thorium. Investigating this complex procedure, we demonstrate the observation of single $^{232}$Th$^+$ ions trapped, embedded into and sympathetically cooled via Coulomb interactions by co-trapped $^{40}$Ca$^+$ ions. Furthermore, we discuss different options for a non-destructive identification of the sympathetically cooled thorium ions in the trap, and describe in detail our chosen experimental method, identifying mass and charge of thorium ions from the positions of calcium ions, as their fluorescence is imaged on a CCD camera. These findings are verified by means of a time-of-flight signal when extracting ions of different mass-to-charge ratio from the Paul trap and steering them into a detector.