This year’s European Frequency and Time Forum (EFTF) is already the 32nd edition of this conference series and will take place in Torino, Italy, during the upcoming week. This conference is one of the global gathering of researchers involved with the measurement and distribution of time and frequencies; the conference webpage can be found here. Each year, a European Time and Frequency Award is given to an outstanding researcher to recognize his or her work. This year’s EFTF Laureate is Ekkehard Peik (PTB), one of the driving forces behind the nuClock project. He will receive the award “for seminal contributions to single-ion optical frequency standards and high-precision spectroscopy thereby establishing most stringent limits on possible variations of fundamental constants”. Congratulations to Ekkehard for being this year’s EFTF Laureate!
The Th-229 nuclear isomer has been around for more than 40 years already, but two of its main properties, namely its energy and lifetime, are known only with very large error margins. While there is no experimental value for the bare isomer lifetime at all, there is at least some consensus on the isomer energy (somewhere between 6.3 and 10 eV). In their recent study (now available on the arXiv preprint server here), the Russian group at MEPhI suggests an energy of 7.1 eV and a lifetime of about half an hour for the bare isomer. These values are in agreement with all recent experiments, which is very good news. And it’s the first time since the Beck et al. measurement (more than 10 years ago !) that a research team dares to put forward a value of the isomer energy. Let’s hope that an independent experiment using a different approach will soon be able to confirm this value. Congratulations to the Russian team for their work!
The TU Wien group and the Metrology Light Source (MLS) in Berlin (a part of PTB) have joined forces to directly excite the Thorium isomer in the VUV. In a measurement campaign about a year ago, Th-229 doped crystals were illuminated by tunable undulator radiation at the MLS facility. A wavelength region between 124 and 240 nm was probed (5.2 to 10 eV) at illumination & detection times between 30 and 600 seconds. The result of this study has now been made available on the arXiv preprint server (find the publication here).
In short, massive photoluminescence masked any possible gamma emission of the isomer. Three different types of photocathodes were used (Cs-Te, Cs-I, diamond), where only the Cs-I version was capable of supressing the photoluminescence at longer wavelengths to a degree that allowed researchers to draw any conclusions on the energy and lifetime of the isomer. Assuming radiative decay as the only decay channel, an isomer lifetime between 0.2 and 1.1 seconds can be excluded for an isomer energy between 7.5 and 10 eV (the region of highest sensitivity of the Cs-I detector). This study complements earlier work in the group of Eric Hudson at UCLA, where a parameter region of similar energy, but longer lifetime was excluded. Also, the results of this study are in agreement with the work at LMU.
Although not successful in this campaign, the results give direct input to the design and protocol of the next round of measurements.
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 (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”.
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.