News

Theoretical work on the hyperfine structure of Th-229m

The hyperfine interaction of the nucleus with the electron shell leads to energy shifts of electronic transitions that are easily accessible to laser spectroscopy. Now that Th-229m ions in the isomeric state became available at the LMU experiment, the PTB group performed laser spectroscopy on these ions and compared the spectra with Th-229 ions in the nuclear ground state. These measurements allowed the two teams to learn a lot about the properties of the nucleus when it’s excited to the isomeric state (see the paper here). The value of the observed nuclear magnetic dipole moment, however, disagrees massively with previous calculations: this clearly calls for a clarification on the theory side.

New calculations performed by the PTB group can reproduce the experimental values to an excellent degree and thus help to better understand the Th-229m nucleus. This work has now been published here.

Thorium research at the DPG Spring Meeting

You want to meet the nuClock people and learn about the latest state of current research? Then head for the DPG meeting in Erlangen, which takes place March 5 – 9. Peter G. Thirolf of LMU Munich will give a plenary talk on Monday morning, and an entire session (A/Q 44) on Friday will be devoted to precision spectroscopy of nuclear systems. There will be five talks of nuClock people in this session! More information can be found here.

2017 nuClock meeting in Heidelberg a great success

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

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

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.