2017 nuClock meeting in Heidelberg a great success

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Last week’s 3-day meeting in Heidelberg was a great success. With 33 participants, this was by far the largest nuClock meeting to date (Vienna 2014: 20 participants, Munich 2015: 24, Brussels 2016: 19). Aside from pretty much all the nuClock members, a large crowd of external guests joined the meeting: Koji Yoshimura from Okayama (Japan), Piet van Duppen and Matthias Verlinde from Leuven (Belgium), Nikolay Minkov from Sofia (Bulgaria), Rukang Li, Xiaoyang Wang, Mingjun Xia, and Lijuan Liu from Beijing (China), Mustapha Laatiaoui and Christoph Düllmann from GSI, as well as the local MPIK fellows José Crespo, Sergey Eliseev and Klaus Blaum. A total of 22 talks were given, all of them showing exciting results or new ideas that will be published in the near future. Many thanks to Adriana for hosting the meeting!

The next nuClock meeting will take place in Bad Honnef, Germany, on July 9 – 12, 2018. This is going to be a large conference (approx. 100 participants), entitled “WE-Heraeus-Seminar: Novel Optical Clocks in Atoms and Nuclei”.

Group photo of the nuClock team 2017. Last row, left to right: Koji Yoshimura (Okayama), Rukang Li, Mingjun Xia, Lijuan Liu, and Xiaoyang Wang (all Chinese Academy of Sciences, Beijing), Przemyslaw Gkowacki and Maksim Okhapkin (both PTB). Third row: Andreas Fleischmann (KIP Heidelberg), Sarina Geldhof and Ilkka Pohjalainen (both Jyvälylä), Piet van Duppen and Matthias Verlinde (both KU Leuven), David-Marcel Meier (PTB), José Crespo (MPIK Heidelberg), Lars von der Wense (LMU Munich), Pavlo Bilous (MPIK Heidelberg). Second row: Jürgen Stuhler (Toptica AG), Peter G. Thirolf and Benedict Seiferle (LMU Munich), Johannes Weitenberg (MPQ Garching), Georgy Kazakov (TU Wien), Iain Moore (Jyväskylä), Nikolay Minov (Sofia). First row: Simon Stellmer (TU Wien), Annkatrin Sommer (Toptica AG), Thorsten Schumm and Kjeld Beeks (TU Wien), Brenden Nickerson (MPIK Heidelberg), Ekkehard Peik (PTB), and Adriana Palffy (MPIK Heidelberg). Not pictured here: Christoph Düllmann (GSI & Mainz).

nuClock growths with the addition of associate members

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The core of the nuClock project is formed by eight European groups, which receive funding from the European Union. The nuClock team seeks to attract more and more scientists into the field of research on Th-229, and to foster communication and synergies among all Thorium groups world-wide. In order to increase the visibility of strong links to partners outside of the project core, we have established a group of so-called nuClock associates. These people or research groups are on the nuClock mailing list, they are invided to all meetings and are formally tied to the nuClock project. Our newly appointed associates are:

  • Piet van Duppen, KU Leuven (experimental search for the Th-229 isomeric transition)
  • Christoph E. Düllmann, GSI & Mainz University (radiochemisty and preparation of uranium and thorum samples)
  • Rukang Li & Xiaoyang Wang, Chinese Academy of Sciences, Beijing (growth of KBBF crystals)
  • Koji Yoshimura, Okayama University (X-ray excitation of Th-229 at SPring-8)
  • Thomas Stöhlker, Jena (X-ray lenses)
  • Atsushi Yamaguchi, RIKEN (Th-229 and other optical clocks)
  • José Crespo, Heidelberg (EBITs, highly charged ions for clocks)
  • Kerstin Ergenzinger, Berlin (artist within the FEAT project)

Advances in HHG laser development

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Tunable narrow-linewidth lasers, as required for precision spectroscopy, are available only in the visible and infrared wavelength ranges, but not in the VUV range (below 200 nm). Unfortunately, many of the most important transitions lie in this specific wavelength range: building a laser for the VUV range would allow one to perform spectroscopy on the Lyman-alpha transition in hydrogen (121 nm), on He+ ions (60 nm), on a variety of highly charged ions which are relevant for cosmology, and (of course), the Th-229 nuclear transition.

Such lasers build on high-harmonic generation in a gas jet, which is quite an inefficient nonlinear process. As a consequence, lasers with both high average power and high peak power (short pulses) are required. The combination of short pulselength, high repetition rate, and high average power is hard to fulfill. Researchers at MPQ in Garching now made an important step forward: Instead of using Ti:Sa lasers (which are common in the field), they used a pulsed Yb-doped laser at 370 W average power, however with a comparably long pulse length of 860 fs. Using a scheme called multi-pass cell spectral broadening (MPCSB), they were able to shorten the pulse length to 115 fs, which is an increase in peak power by a factor of about 7. The specific laser developed here will be used for spectroscopy of He+ ions, but the technology could also be transferred to a laser system dedicated to Th-229 research.

The work has recently been published with Optics Express and can be found here.

Breakthrough: The first optical spectroscopy of Th-229m ions

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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.

Theory paper on laser-induced de-excitation of the isomer

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The exact energy of the isomer is still unknown, but there is good news: the recent experiments at LMU in Munich have shown that a 2 percent fraction of U-233 recoil ions are in the isomeric state. Such ions can now be used for spectroscopy. In a recent publication, researchers from MPIK in Heidelberg and PTB in Braunschweig suggest a new approach to measure the isomer energy. In a so-called LIEB process (laser-induced electron bridge), an electron combines the energy of the isomer together with the energy of a photon of the excitation laser to resonantly populate an excited electronic state. From here, it may decay down again into a lower electronic state. Such laser-assisted excitation increases the nuclear decay rate by orders of magnitude. The manuscript is now available here on the arXiv.