nuClock FEAT prepares for first exhibition

A few months ago, nuClock joined the FEAT initiative to explore the interplay between arts and sciences. The cooperation with artist Kerstin Ergenzinger of Berlin will come to life at the upcoming Ars Electronica exhibition in Linz, Austria. Ars Electronica is the world’s largest and most influential exhibition of digital art. Held annually since 1979, it is set up as a 4-day festival with a focus on the interface between arts, electronics, and society. The motto of this year’s meeting is “RADICAL ATOMS and the alchemists of our time”: what a picture-perfect setting for our nuClock work! The event will last from September 8 through 10, and is expected to attract 600 exhibitors and 100.000 visitors.

nuClock will be featured on the opening day, Thursday Sept. 8. At first, Kerstin Ergenzinger and Simon Stellmer will have an interview with Annick Bureaud, which will later morph into a video blog. Later, the two will present the concept of nuClock’s first piece of art: an installation that builds on earlier “navigating noise” artwork. In addition, Thorsten Schumm will join a podium discussion on arts and sciences.

To prepare for the exhibition, Simon Stellmer spent a two-day visit at Kerstin Ergenzinger’s studio in Berlin last week. In turn, Mrs. Ergenzinger visited the MLS facility in Berlin-Adlershof, where a joined TU WIEN / PTB experiment is currently prepared to search for the Th-229 isomer transition. This measurement – detecting a faint signal on top of large background noise – is the inspiration for the joint artwork.

ICAP 2016 in Seoul

The 25th International Conference on Atomic Physics (ICAP) is currently taking place in Seoul. This series of conferences is probably the largest, oldest, and most prestigeous series of conferences in the field of atomic & quantum physics. Held only every other year, this year’s conference attracted some 500+ participants to the main conference, but also to the various satellite meetings arranged around it. The nuClock consortium is represented by four groups: PTB (Ekkehard Peik), MPQ (Thomas Udem), TU Wien (Simon Stellmer), and Toptica Photonics (Stephan Falke). They are joined by a number of other thorium enthusiasts: Dave DeMille (Yale), Oscar Herrera-Sancho (ex PTB, now Innsbruck & Costa Rica), Atsushi Yamaguchi (ex PTB, now RIKEN), as well as Noboru Sasao, Takahiko Masuda, and Hideaki Hara (all Okayama University). You asked about the venue? It’s the famous COEX center, right in the middle of the world-famous Gangnam district. More information can be found at the conference webpage.

nuClock yearly meeting 2016

Already one year had passed since the start of the nuClock project, so the time was about right for the first annual meeting. Very much in the spirit of the 2015 kick-off meeting in Munich, this year’s meeting, held in Brussels, featured a number of external guests from outside the consortium. nuClock was very pleased to welcome representatives of RIKEN (Japan), GSI (Germany), and NPL (UK). Monday and Tuesday of this week (July 18/19) were filled with a series of talks, nicely interlaced with delicious food. (From another perspective, one might also say that an astonishing series of meals & drinks got interrupted by talks every once in a while.) The yearly meeting was concluded by a visit to the European Parliament, where a private tour allowed us to peek into European politics. Thanks to everyone who participated! The next yearly meeting (summer 2017) will be hosted by the MPIK Heidelberg group.

brussels group photo

Group photo taken at the 2016 yearly meeting. Front row: Giuseppe Larusso (NPL), Stephan Schneider (Vienna), Sarina Geldhof (Jyväskylä), Thorsten Schumm and Simon Stellmer (Vienna). Second row: Brenden Nickerson (Heidelberg), Ekkehard Peik (PTB), Andreas Fleischmann (Heidelberg), Georgy Kazakov (Vienna). Third row: Mustapha Laatiaoui (GSI), Pavlo Bilous (Heidelberg), Matthias Scholz (TOPTICA). Fourth row: Atsushi Yamaguchi (RIKEN), Johannes Thielking (PTB), Christian Enss (Heidelberg). Fifth row: Lars von der Wense and Peter Thirolf (LMU), Ilkka Pahalainen (Jyväskylä), Jürgen Stuhler (TOPTICA).

The check meeting, which concluded the first funding period, was scheduled on the day following the yearly meeting. Two representatives of each consortium partner took to the Research Executive Agency in Brussels to report on the work performed during the last year. To cut the entire review process short: the board of project monitors was pleased 🙂


nuClock visiting the European Parliament (notice the UK flag on the far right).

facebook 2.0

nuClock has always had a facebook page, but well… we never really used it. This has changed now! Sarina Geldhof has taken over our facebook page, which is kind of equivalent to fixing a rocket booster to your 35-year old Vespa. Visit our page, like and share what we do, and stay up to date with Sarina’s (almost) daily posts!

Two papers in two days

Two nuClock papers surfaced this week: On Monday, the conference proceedings of last year’s FS&M symposium in Potsdam appeared, with a contribution by the Vienna group (link). The proceedings are free to download.

Then on Tuesday, recent work on U-233 doped crystals appeared with Phys. Rev. C (link). This paper had kind of a rough start, cycling through half a dozen review rounds with nearly the same number of referees absorbed. Some of the referees’ remarks were not about the content itself, but rather as to whether the “Thorium topic” is interesting at all. Our most favorite comment by one of the referees (slightly rephrased here): “So a direct observation of the decay of the isomer and determination of its half-life are rather a technical challenge than a highlight in nuclear structure physics. (…) The paper should therefore be deferred to a more technical journal.“. Let’s recall that APS already did publish three “First observation of the thorium isomeric transition” papers, all of which turned out to be false very soon after… Read a beautiful comment by Reinhard Werner (link) very much along these lines.

The Phys. Rev. C paper explores a rather new idea to measure the isomer energy by optical spectroscopy, first mentioned a couple of years ago by Eric Hudson’s group (link to the paper). Past experiments used U-233 recoil nuclei adsorbed on a surface (but suffered from low count rates) or synchrotron radiation on crystals (but could never be sure if the isomer was populated at all). The new approach combines the benefits of these experiments: a reliable source of isomeric nuclei (U-233 decay) combined with a bulk crystal (effectively going from 2D to 3D). The expected count rate is orders of magnitude larger compared to experiments using the accumulation on surfaces, and might yield a signal within a few weeks of measurement time. Internal conversion is still expected to be the biggest spoiler, especially since the position or state of the Th-229m ion cannot be controlled.

Global response to the recent LMU Nature paper

The recent publication on the first direct detection of the isomeric state (link to the Nature paper) was accompanied by a number of press releases, which were picked up and posted on a number of online platforms. With a delay of a few weeks, we are very pleased to find articles appearing all around the world: articles that do not just copy/paste the press releases, but reflect or comment on our work from a new perspective. Here, we will highlight two of them: is a well-known blog that highlights outstanding scientific results in physics. The latest article covers work of the LMU group (link).

Quite surprisingly, the Indian newspaper “The Statesman” recently featured the LMU work in an extensive science article. This newspaper, with a circulation of 180’000, is one of the major newspapers in West Bengal, India, and appears in English. The online version can be found here.

Chad Orzel of Union College wrote a comment for Forbes magazine, please find the link here.

Welcome Brenden Nickerson!

The nuClock family keeps growing! Today, we welcome Brenden Nickerson to the team. Brenden joined Adriana Palffy’s group in Heidelberg and will strengthen the theory support of the consortium. Good luck for your work, Brenden!

Brenden Nickerson

Brenden Nickerson, the latest addition to the nuClock consortium.

Detection of the nuclear clock transition: Media coverage

The recent LMU publication led to a broad media coverage of our work! We will try to keep track of them:

Original article and commentary:

Nature article, link
Nature News & Views commentary by Marianna Safronova, link

Press releases:

LMU press release in German and English
Mainz university press release in German and English
GSI press release in German and English

German press:, link und link
Welt der Physik, link
Wissenschaft aktuell, link, link, link

International press:

Physics World (IOP), link, link
inverse, link, link, link, link
Oxford virtual, link, link
Alpha Galilieo, link
Eurekalert!, link
Science Daily, link
Science Newsline, link
ScienMag, link
Odd Onion, link
Public., link
Science and Technology Research News, link
techradar, link
Bandwidth Blog, link
in Swedish: link
in Russian: link

First direct detection of the Th-229 isomer: LMU work published with Nature!

A giant leap in the development of a nuclear clock: the LMU group has directly observed the de-excitation of the Th-229 isomer via internal conversion. This is the first direct proof of the existence of the isomeric state. Today, this work has been published with Nature. Let’s look at the experiment more closely:

The naive way to prove the existence of the isomeric state in Th-229 would be a detection of the VUV gamma that is emitted as the isomer decays into the ground state. This approach has been followed by a dozen of past experiments, and by a handful of ongoing experiments. So far, all of these experiments could not observe a signal, or were not able to unambiguously attribute the observed signal to the isomeric decay. Various methods have been employed to populate the isomer in the first place: α-decay of U-233 (this is the most commonly used method), β-decay of Ac-229, optical excitation by means of various light sources, electron bridge processes, and a few more. Very recently, a number of very well-designed experiments, employing either the U-233 decay or optical excitation by synchrotron radiation, were unsuccessful in finding evidence of the isomer. Besides the trivial explanations for these null measurements (“The isomer does not exist.” and “Lifetime and/or energy of the isomer are very different from what is currently believed.”), it is the internal conversion (IC) channel that can inhibits the emission of an optical signal. In the IC process, the isomer would release its energy not via emission of a gamma particle, but would transfer its energy to electronic excitations of the thorium ion itself, or neighboring ions or atoms (such as atoms of the substrate or crystal material that the thorium ion is bound to). In such a process, a low-energy electron would be released. It is precisely this decay channel (and not the optical one) that the LMU group used in their experiment.

The LMU group chose the following strategy: isomer population via α-decay of U-233, combined with isomer detection via observation of the electron released during IC de-excitation. The isomer production part is well-established and quite robust: a very thin layer of U-233 is deposited onto a disk. As the U-233 undergoes α-decay, the Th-229 daughter nucleus gets a momentum kick equivalent to an energy of up to 80 keV, which propells it a few 10 nm through the uranium material. If the thickness of the overlaying material is smaller, the nucleus will reach the free space. It can then be trapped in an ion trap, deposited onto a catcher plate, or guided to a detector. The fraction of daughter nuclei that appear in the isomeric state Th-229m (as opposed to the nuclear ground state Th-229g) is about 2 percent.

The detector of choice for the observation of single electrons are multi-channel plates (MCPs). These devices amplify a single electron to an electronic signal containing an avalance of millions of electrons. Naively, one would proceed to simply place the U-233 source in close proximity of the MCP and count the electrons emitted during de-excitation of the isomer. Unfortunately, this approach does not work, as any ion striking the MCP with an energy of tens of keV will produce a “click” (it’s all radioactive material, after all). The ions thus need to be slowed before being deposited carefully onto the detector. Building a device that could slow down the Th-229 ions (and filter out all other ions) is no mean feat and took the LMU group half a decade.

The fate of a Th-229m ion is the following: After on average 160.000 years, it is born through α-decay of a parent U-233 nucleus. It travels through a few nm of uranium, reaches the vacuum of a large vessel, and is slowed and buffer-gas cooled by helium. The thermalized ions are extracted through a nozzle into an ion guide and further into a quadrupole mass-separator, where ions with different mass numbers are removed from the beam. The Th-229m ion is then gently deposited onto the surface of an MCP, where the residual impact energy is carried away by phonons. The ion quickly rips off electrons from the surface to neutralize. Now that the Th-229m atom has become neutral, the isomer energy is above the first ionization threshold: the isomer de-excites by transferring its energy to the least bound electron, which leaves the Th-229 atom with an excess kinetic energy of at most a few eV. While the Th-229 ion absorbs yet another electron from the surface to neutralize again, the emitted electron starts a signal cascade in the MCP that grows to form a mature avalanche of electrons. These impinge onto a phosphor screen, where they are converted into visible photons. These in turn are imaged by a CCD camera. The rate of such events is low, but integration over half an hour yields a sufficiently large signal.

The LMU group then performed a myriad of cross-checks to exclude literally all other possible origins of the observed signal. The signal appeared only for different charge states of Th-229 and for U-235, which is also known to possess an isomer. It did not appear for any other isotope, and it did not appear with the U-233 source replaced by a U-234 source. The signal could not be matched to any α- or β-decay, as these generate clearly different images on the detector.

This experiment thus adds a very valuable piece to the mosaic of investigations of the Th-229 isomer. While a number of past experiments have inferred the existence of the isomer from indirect measurements, this is the first direct observation of single property of the isomer: its de-excitation via IC. Many other properties are still to be determined, namely its energy, its lifetime, and the ability to drive the isomer transition optically. Concerning energy and lifetime, the LMU experiment was not aimed to improve existing values, but it is in agreement with them. The inferred energy falls between the first and third ionization threshold (between roughly 7 and 18 eV, where the consensus on the energy is currently 7.8(5) eV). By storing the Th-229 ions for a little while before detection, a lower bound of about one minute could be placed on the lifetime in vacuum (current estimate: about 15 minutes). Adaptations of the experiment will allow to measure both energy and lifetime more precisely.

Radiochemistry meeting in Vienna

We are currently preparing for an informal workshop concerning the radiochemistry of thorium and uranium isotopes. The workshop will be held on April 28 in Vienna, mainly featuring scientists from the NPL radioactivity group (UK) and the Atominstitut in Vienna. Please contact us if you would like to join!