Storage and cooling of thorium ions from radioactive decay

Triply positive charged ions of the isotope Th-229 (Th3+) are of particular interest as the basis for a high-precision optical clock. As the isotope is radioactive, it is only available in very small quantities and conventional methods for realising an ion source (for example by vaporisation) are not applicable in this case.
The PTB (Physikalisch-Technische Bundesanstalt; Federal Physical-Technical Institute, the National Metrology Institute of Germany) has now developed an apparatus for generating, cooling and storing Th3+ as recoil ions from the radioactive decay of U-233. A thin film of U-233, produced by our group in Mainz, emits the desired ions into the vacuum, but with a high initial energy of more than 80 keV. By decelerating in high-purity helium gas, it is possible to collect the ions and transfer them to an ion trap. There they are stored together with Sr+ ions, which are cooled to a temperature in the millikelvin range using laser light. The coupling with the Sr ions also reduces the energy of the Th3+ to a value that is about 11 orders of magnitude below the starting energy after the U-233 decay. The stored Th3+ ions are now available at low energy for precision measurements of their resonance frequencies. This provides a basis for studying the properties and structure of this unusual isotope.

The work was carried out in cooperation with Johannes Gutenberg University Mainz and the GSI Helmholtz Centre for Heavy Ion Research, where the U-233 sources were produced.
The results have now been published in the journal Physical Review A (, where they were highlighted as an "Editor's Suggestion".

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