TACTICa

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. 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. 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 ThF2+. 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.

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

 

multiply-CHArged Radioactive Molecules (mCHARM)

What began as a simple test stand to optimize thorium ablation parameters has evolved into a unique platform for producing and studying multiply charged radioactive molecules. In the mCHARM project, we extend our focus beyond thorium toward the lighter actinides, exploring the controlled generation of molecular ions with charge states above 1+. Using laser ablation under high fluences, we systematically vary target materials to form actinide molecules at the edge of chemical stability.

Ultimately, mCHARM aims to study these exotic ions spectroscopically in order to investigate symmetry violations beyond the Standard Model.
mCHARM is a collaborative effort between HIM, GSI, MPI-K, MIT, and JGU.

 

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

 

Publications

  1. Jonas Stricker, Jean Velten, Valerii Andriushkov et al.
    Laser-fluence-dependent production of molecular thorium ions in different charge states for trapped-ion experiments
    Phys. Rev. A. 112, 012821 (2025).
  2. Paul Fischer, Jonas Stricker, Christoph E. Düllmann, Dennis Renisch, and Lutz Schweikhard
    Gas-phase thorium clusters from laser ablation suggest magicity of Th+13
    Eur. Phys. J. D (2025) 79 :30.
  3. Paul Fischer,  Jonas Stricker, Christoph E. Düllmann,  Dennis Renisch, Lutz Schweikhard, and Christian Tantardin
    Gas-phase thorium molecules from laser ablation
    Phys. Rev. Res. 6, 043317 (2024).
  4. Marvin Gajewski, Wenbing Li, Sebastian Wolf, Walter Hahn, Christoph E. Düllmann, Dmitry Budker, Giovanna Morigi, and Ferdinand Schmidt-Kaler
    Fluorescence calorimetry of an ion crystal
    Phys. Rev. A 106, 033108 (2022), arXiv:2204.06513 .
  5. 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. 24, 043028 (2022);  arXiv:2109.00575 .
  6. 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).
  7. 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 .
  8. 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 .
  9. 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).