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!

Upcoming conferences: Meet the nuClock people!

You would love to meet some of the nuClock folks in person? This is where you will get a chance:

DPG Frühjahrtagung in Hannover, Feb. 29 – March 04, 2016  (AMO physics)


  • Mo, 11:00     MS 1.1    “Towards a nuclear clock: On the direct detection of the Thorium-229 isomer” by Lars von der Wense (Peter G. Thirolf’s group at LMU Munich)
  • Mo, 16:00     A 7.5     “Performance and readout of state-of-the-art MMC detector arrays” by D. Hengstler (Christian Enss’ group at KIP Heidelberg)
  • Thu, 15:15 Q 56.4 “An optomechanical interface bridging x-ray and optical photons” by Adriana Pálffy (MPIK Heidelberg)


  • Wed, 16:30     A 30.3     “Metallic Magnetic Calorimeters for high resolution X-ray spectroscopy” by M. Krantz (Christian Enss’ group at KIP Heidelberg)
  • Wed, 16:30     A 30.11     “Search for optical excitation of the low-energy nuclear isomer of 229Th” by Johannes Thielking (Ekkehard Peik’s group at PTB Braunschweig)
  • Wed, 16:30     A 30.10     “Towards a nuclear clock based on 229Th: Internal conversion rates for Th ions” by Pavlo Bilous (Adriana Pálffy’s group at MPIK Heidelberg)
  • Wed, 16:30     A 30.24     “Polar-maXs: Micro-calorimeter based X-ray polarimeters” by Christian Schötz (Christian Enss’ group at KIP Heidelberg)

Exposition of companies:

  • Mo through Thu: TOPTICA AG, Gräfelfing

DPG Frühjahrtagung in Darmstadt, March 14 – 18, 2016  (nuclear physics)


  • Mo, 14:30     HK 6.2    “Direct detection of the thorium-229 isomer: Milestone towards a nuclear clock” by Lars von der Wense (Peter G. Thirolf’s group at LMU Munich)
  • Mo, 15:45     HK 6.6    “Using 233U-doped crystals to access the few-eV isomeric transition in 229Th” by Simon Stellmer (Thorsten Schumm’s group at TU Wien)


  • Mi, 18:30     HK 45.10     “Prospects for an energy determination of the 229mTh nuclear isomer via IC electrons” by Benedict Seiferle (Peter G. Thirolf’s group at LMU Munich)

European Fequency and Time Forum 2016 (York, UK) April 4 – 7, 2016

This conference, held in the beautiful city of York, is co-organized by Ekkehard Peik (PTB), acting as Chair of the Executive Committee. Simon Stellmer and Georgy Kazakov of Vienna will also be around for a chat. See the conference webpage for more details.

Laser2016 (Poznan, Poland) May 16 – 19, 2016

Invited talks by Ekkehard Peik (PTB) and Iain Moore (Jyväskylä), more details on the conference webpage.

25th ICAP conference in Seoul (Korea), July 24 – 29, 2016

A number of contributions from the nuClock community, details will be announced at the conference webpage.

Merry Christmas

A peaceful and joyful holiday season to all of you, your families and beloved ones!

Beamtime at SPring-8

Exciting thorium-related research activity is currently going on in Japan! Out of a multitude of approaches to populate the isomeric state, a Japanese/Austrian initiative chose “optical” pumping with X-rays. Synchrotron radiation at SPring-8 is used to excite the Th-229 nucleus into the second excited state, located at 29 keV. This state may de-excite back into the ground state, or, with a probability of above 90%, fall into the isomeric state.

An unusually long beam time of 18 shifts (6 days in total), has been allocated to advance on this project. The currently ongoing measurement campaign is subdivided into two major parts: (i) finding the 29-keV nuclear resonance, and (ii) detecting the VUV gamma emitted upon de-excitation of the isomer.

The first experiment employs a “spotted” target with a layer of Th-229 deposited onto a substrate. The 29-keV gamma that is re-emitted as the nucleus de-excites into the ground or isomeric state is detected with a super-fast MCP.

The second experiment uses thorium-doped crystals produced by the Vienna group. The X-ray beam, tuned to the nuclear resonance, pumps population from the ground state into the isomeric state. After a few minutes, the beam is turned off, and the emission of the crystal is measured with a PMT and a VUV spectrometer.

The research is guided by Koji Yoshimura from Okayama University. Further partners include Kyoto University, Tohoku University, Osaka university, RIKEN (including Atsushi Yamaguchi, who had previously been working at PTB), and SPring-8. Measurements will last from December 14th through 20th.