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!

Thorium on steroids: A new proposal by E.V. Tkalya

Many research groups around the world try to get an “optical” handle on the Th-229 isomeric state, but it turns out that the detection of optical photons emitted upon de-excitation of the isomer is no mean feat. As an alternative, one might shift gears and aim to detect the electron that is emitted during internal conversion (IC) of the isomer. Such an approach has been suggested by the LMU group at last week’s DPG conference in Darmstadt, Germany.

The IC electron would have a kinetic energy of a few eV only: an energy scale that nuclear physicists might feel uncomfortable with. “Is there a way to boost this energy?”, E.V. Tkalya (a well-known expert in the field of Th-229 research, based in Moscow) asked himself. Obviously, the few-eV isomer energy cannot be changed, but he found a different way: in a recent publication (see the preprint on the arXiv), he suggests magnification the ground and isomer states’ hyperfine structures into the 100 eV range. The required magnetic field is generated by substituting the innermost 1S electron of the Th-229 atom by a muon. The muon’s orbit is effectively within the nucleus, generating a magnetic field of a few 10 GT. The lifetime of the muon would be on the order of 100 ns.

The enormous magnetic field splits the ground state into a hyperfine doublet with an energy gap of some 350 eV. The hyperfine splitting of the isomer would be only a few eV. As a consequence, the upper ground-state hyperfine state appears above the isomer doublet, allowing for a bizarre scenario: the ground state may populate the isomer by simple relaxation! As the muon disappears again after 100 ns, the nucleus might remain in the isomeric state. It is estimated that on today’s existing muon factories (e.g. PSI in Switzerland), on the order of 10 nuclei in the isomeric state could be produced per second.

Apart from shifting the energy of the IC electrons into the 100 eV range, there is another appealing asset to this approach: during the (admittedly short) lifetime of the muon, the hyperfine structure of the isomer is about 5 eV, and could be driven by laser light. Hyperfine transitions are thus pushed from the microwave into the optical domain.

Time will tell if the Th-299 research, technologically quite involved already, can benefit from muonic atoms.

nuClock on air

What an exciting day for nuClock: Following his talk at the DPG Spring Meeting in Hamburg, Lars von der Wense of LMU Munich made an appearance on radio Deutschlandfunk. He was interviewed in a science show and explained the concepts, prospects, and applications of a nuclear clock. The interview is in German, and the audio file can be found here. According to a recent media analysis, more than 6 million people regularly tune in to radio Deutschlandfunk. Congratulations to Lars!

Also today, Jun Ye of JILA visited the Vienna group. Jun is leading research experiments is many different fields, among them optical lattice clocks, ultracold molecules, and XUV frequency comb spectroscopy. Visit his group webpage to find out how his group constantly pushes the border of what’s technologically feasible. Jun, thanks for spending the day with us!