Researchers at LMU Munich have put together a new review paper on the history of work on the Th-229 isomer, its current status, as well as potential implementations of a nuclear clock. The paper also investigates the properties of other candidates (aside from Th-229) to be used for a nuclear clock. It was assembled in connection with a talk given by Lars von der Wense at last year’s 175th anniversary of the Mendeleev All-Russia Research Institute of Metrology (VNIIM) in St. Petersburg, and can be downloaded here.
The nuClock consortium is set to organize a conference entitled NOCAN: Novel Optical Clocks in Atoms and Nuclei. The goal of the seminar is to discuss conceptually novel approaches to optical frequency standards and clocks. Despite the remarkable precision already realized in current atomic clock worldwide, there is a wide range of concepts for next-generation devices, rooted in very different physical systems. The aim of the seminar is to provide a comprehensive overview on the current state of discussion and shape a community. Also, we want to identify and discuss new applications of precision frequency and time standards, in particular regarding constraining possible variations of fundamental constants, clock-based geodesy, and gravitational wave detection.
Registration for the conference starts now: please find the webpage here, which includes a registration form and a template for submission of your abstract. We are looking forward to welcome you at the conference!
The list of already confirmed invited speakers:
· Ekkehard Peik
· Peter Thirolf
· Marianna Safronova
· Jose Ramon Crespo Lopez-Urrutia
Talks on the nuclear clock
· Koji Yoshimura
· Marcin Piotrowski
· Petr Borisyuk
· Thorsten Schumm
· Christian Enss
· Christian Schneider
· Eric Hudson
· Adriana Palffy
· Matthias Verlinde
Talks on novel approaches, applications, theory
· Julian Berengut
· Atsushi Yamaguchi
· Nils Huntemann
· Andrew Ludlow
· Michal Zawada
· Victor Flambaum
· Tanja Mehlstäubler
· Pacôme Delva
· David Champion
· Jean-Lautrier Gaud
Another five speakers for “Hot Topic Talks” will be selected from the submitted abstracts.
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.
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.
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 (Romania), 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”.
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)
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.
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.
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.
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!