So far, all experiments that characterized the Th-229 nuclear isomer employed nuclear physics techniques: gamma spectroscopy, alpha spectroscopy, detection of electrons, coincidence schemes, and the like. For the nuclear optical clock, however, technology out of the quantum optics toolbox will be requires, such as lasers, optical detection, and precision spectroscopy. A recent experiment by the PTB, LMU, and GSI groups now made a huge step into this direction: they performed the first laser spectroscopy of electronic states in Th-229m ions.

The experimental realization was truely a team effort: at first, the hyperfine structure of Th-229 in its nuclear ground state was measured at the thorium ion trap at PTB. Then, all the lasers and required optics were brought to LMU Munich to measure the combined Th-229 + Th-229m in the LMU ion trap. A U-233 recoil source was used to produce the Th-229m nuclei in the isomeric state. The combined spectrum of Th-229 and Th-229m clearly showed additional peaks that were not present in the pure Th-229 measurements. The hyperfine structure of two different electronic levels was investigated, and the number of additional peaks was sufficient to determine the A and B parameters for these two levels in Th-229m. A comparison with the Th-229 nucleus then allowed the authors to calculate the magnetic moment of the Th-229m nucleus. The value of -0.37(6) µ_N is about five times larger than the previously accepted value derived  from the Nilsson model. In addition, the quadrupole moment of the isomer was determined to be Q=1.74(6) eb. From this value, one can infer that the geometric shape of the nuclear charge distribution of the isomer is very similar to the one of the nuclear ground state. The difference in the mean-square radii of the ground and isomeric states is calculated as 0.012(2) fm^2. With these values, we have a very clear image of what the isomer looks like.

Following the first direct detection of the isomeric state and the determination of the isomer lifetime unter internal conversion decay, this work is the third major breakthrough within the nuClock project. The corresponding publications can now be retrieved from the arXiv preprint server here.

Combined hyperfine structure of the transition at 1164 nm, connecting the 20711 and 29300 electronic states in Th2+. The cloud of ions, containing 2% ions in the isomeric state, are used for spectroscopy. The four small peaks, labelled with quantum numbers and highlighted in cyan color, belong to the isomeric state Th-229m. These peaks are not present in a pure sample of Th-229 with all ions in the nuclear ground state.

A handful of experiments have tried optical excitation of the isomer already, unfortunately without success. All of these experiments searched for delayed fluorescence in the optical domain as the smoking gun of an excitation of the isomer. The main obstancle in these experiments can be summarized as follows: The transition linewidth is teeny-weeny small, probably about 0.001 Hz, but the linewidth of excitation sources is very broad, about 100,000,000,000,000 Hz. So it’s very unlikely to excite the nucleus. The small excitation probability can be offset by using many many nuclei, say 10^15 nuclei. Such large numbers of atoms need to be cast into some solid form, either as a metal, a dopant into a some sort of host material, or a layer attached to some underlying material. But once the isomer is confined in a solid, it tends to undergo internal conversion: it will de-excite by emitting an electron rather than a photon. This process might explain why previous experiments, which searched for an optical signal, were not successful.

Now, Lars von der Wense (LMU Munich group) proposes to use the best of two worlds: optical excitation via lasers, detection via electrons. There do exist pulsed lasers with sufficiently small linewidth and sufficiently large power to make this approach feasible. In addition, the detection of the isomer via spectroscopy of the IC electron is also well established in Munich: there shall be nothing in the way of this experiment.

This proposal has now been accepted by Phys. Rev. Lett. (find the abstract here) and will be published within the next couple of weeks; the arXiv version can be found here. The list of co-authors includes researchers from 4 out of the 8 nuClock partners: Half the consortium was involved in this proposal.

Congratulations to Lars and the team!

Sitting in your deck chair with nothing to read? We have a solution for you!

A few weeks ago, Francisco Ponce of Lawrence Livermore / UC Davis finished his PhD thesis on the topic “High Accuracy Measurement of the Nuclear Decay of U-235m and Search for the Nuclear Decay of Th-229m”. In his studies, he searched for the IC electron in the de-excitation of the Th isomer, but was not sensitive to timescales in the µs range. Although eventually not successful, the PhD thesis still makes a nice reading. The thesis can be found here.

And while we’re at it, we would like to draw your attention to another study from Jason Burke’s LLNL group, which looked into the distribution of charge states of the Th-229 recoils following the alpha decay of U-233: this work can be found here.

No joke: there exist peer-reviewed journals on arts! The publications look just like science publications: title, author list, abstract, acknowledgements, list of reference… plus the well-beloved discussions with reviewers. Anyways, the most reputated journal covering the interface between science, technology, and arts is probably Leonardo Magazine, published by MIT Press. This is where we published a study on one of the two projects that nuClock associate Kerstin Ergenzinger is currently working on. Please find the link here (or download in the “Publications” section), and an accompanying audio blog post here.

If you want to learn more about the artworks, Kerstin has put a number of photos and videos on her webpage. Kerstin’s work is supported by Daniel Canty, a poet based in Montreal. Check out his webpage as well!

If you want to see the installations in real life, plan for one of the following exhibitions:

A new PhD student joined the nuClock team! Kjeld Beeks from Eindhoven University of Technology just started his PhD in the Vienna group. He will work towards optical excitation and optical detection of the Th-229 isomer transition. Good luck with this challenging work, Kjeld!

Kjeld Beeks joined the Vienna team.