Trapped And Cooled Thorium Ion spectroscopy via Calcium

TACTICa aims at deploying ion trapping techniques like quantum logic spectroscopy to investigate the nuclear structure of different thorium isotopes with respect to precise determination of nuclear moments, hyperfine intervals and isotope shifts. Those fundamental parameters can contribute to the search for BSM physics [1,2]. The experimental base of the project is the trapping of Th ions in a so-called Paul trap, which is developed in the group of Prof. Ferdinand Schmidt-Kaler in the department of physics [3]. Please refer to the TACTICa homepage for more detailed information.

Our group contributes to the collaboration with knowledge and experience in handling radioactive material, so that we will provide the thorium material for TACTICa. But we are not only producing the Th sources, we are also setting up the ion source setups for the experiment. In general, we will follow two different approaches for Th ion production.

Scheme of available U and Th isotopes, that shall be used at TACTICa in the future. Longer living Th isotopes, like 232Th, can be directly ionized by the laser ablation ion source. Shorter living Th isotopes, like 228Th or 229mTh, will be provided by alpha decay of the corresponding U mother in the recoil ion source [5]


Laser ablation

For Th isotopes, where a macroscopic amount of material is available, direct laser ablation is an established method. For TACTICa we are not only interested in simple monoatomic ions, like Th+, but also in certain molecular ions, like ThF+ [4]. For this purpose we are using a laser ablation setup to investigate crucial parameters that have influence on the produced ion species. Possible parameters are laser wavelength, laser power, pressure and of course the initial Th compound that is used as target for the ablation. Different Th salts (nitrates, chlorides, iodides, etc.) are used on the first stage of the studies.

Overview picture of the laser ablation setup (2022), showing the main parts, like the ionization chamber, where the laser hits the installed Th sample, and the attached time-of-flight (ToF) section, that is used for initial ion species identification.
Photo/©: J. Stricker / Univ. Mainz


Recoil ion source

Certain Th isotopes are not available in sufficiently large amounts to provide a target for direct laser ablation, e.g. due to short half-lives. But some of those Th isotopes are accessible as recoil ions by the alpha decay of the according Uranium mother nuclide. One example is the production of the ultra-low energy isomer 229mTh, which is accessible by the alpha decay of 233U. For this purpose a recoil ion source was developed [5], which is based on electrostatic deceleration of the recoils. For successful trapping of ions in a Paul trap, the ions may not exceed a kinetic energy of about 1 keV. In the alpha decay of 233U, the Th recoil gain a kinetic of about 84 keV. Thus, the ions have to be slowed down drastically, which we achieve by biasing the recoil source on a sufficient strong negative potential of about -83 kV. Advantages of this method, compared to other deceleration methods like buffer gas cooling, is the conservation of the initial charge state of the recoil ions [6], opening the possibility to catch Th ions in higher charge states in the Paul trap.

Overview picture of the recoil ion source setup (2022). The 233U source is insulated against ground and can be biased up to -84 kV, so that the released recoil ions are slowed down by electrostatic forces. The recoil ion beam is then guided through a 90° bender, where the Paul trap for catching and studying the Th ions will be installed in the future.
Photo/©: D. Renisch / Univ. Mainz


We offer topics related to TACTICa for research modules, B. Sc. and M. Sc. theses for chemistry and physics students.


[1] V. V. Flambaum, Physical Review Letters 97, 1–3 (2006)

[2] V. V. Flambaum et al., Physical Review A 97, 1–12 (2018)

[3] K. Groot-Berning et al., Phys. Rev. A 99, 023420 (2019)

[4] V. V. Flambaum, Phys. Rev. C 99, 35501 (2019)

[5] R. Haas et al., Hyperfine Interact. 241, 25 (2020)

[6] K. Gunter, et al., Phys. Rev. Lett.16, 9 (1966)


Latest publications

  • Wenbing Li, Sebastian Wolf, Lukas Klein, Dmitry Budker, Christoph E. Düllmann, and Ferdinand Schmidt-Kaler
    Robust Polarization Gradient Cooling of Trapped Ions
    New J. Phys. https://doi.org/10.1088/1367-2630/ac6233 (2022);  arXiv:2109.00575.
  • R. Haas, M. Hufnagel, R. Abrosimov, Christoph E. Düllmann, Dominik Krupp, Christoph Mokry, Dennis Renisch, Jörg Runke, Ulrich W. Scherer
    Alpha spectrometric characterization of thin 233U sources for 229(m)Th production
    Radiochim. Acta 108, 923 (2020).
  • Raphael Haas, Tom Kieck, Dmitry Budker, Christoph E. Düllmann, Karin Groot-Berning, Wenbing Li, Dennis Renisch, Ferdinand Schmidt-Kaler, Felix Stopp, and Anna Viatkina
    Development of a recoil ion source providing slow Th ions including 229(m)Th in a broad charge state distribution
    Hyperfine Interactions (2020) 241:25; arXiv:1911.11674.
  • Karin Groot-Berning, Felix Stopp, Georg Jacob, Dmitry Budker, Raphael Haas, Dennis Renisch, Jörg Runke, Petra Thörle-Pospiech, Christoph Düllmann, and Ferdinand Schmidt-Kaler
    Trapping and sympathetic cooling of single thorium ions for spectroscopy
    Phys. Rev. A 99, 023420 (2019); arXiv:1807.05975.
  • Felix Stopp, Karin Groot-Berning, Georg Jacob, Dmitry Budker, Raphael Haas, Dennis Renisch, Jörg Runke, Petra Thörle-Pospiech, Christoph E. Düllmann, Ferdinand Schmidt-Kaler
    Catching, trapping and in-situ-identification of thorium ions inside Coulomb crystals of 40Ca+ ions
    Hyp. Int. 240, 33 (2019).