News

Thorium research down under

We just learned about a new Thorium experiment, the first one in the Southern hemisphere! At CSIRO/Griffith University, researchers are currently transforming an already existing Yb ion trap to a Th ion trap. What a brilliant idea! The experiment is based in Brisbane (that’s in the South East corner of Queensland), so if you happen do be around or would like to learn more about the experiment, contact Stephen Gensemer or Marcin Piotrowski directly. In a few years’ time, we will have Th nuclear clocks running on all continents to measure how Earth moves through clouds of Dark Matter. 🙂 So you folks better hurry up with your experiment!

Save the date: Thorium-229 conference July 9-12, 2018, in Bad Honnef, Germany

The nuClock team will organize an international conference on all topics related to the Thorium-229 isomer. Today, we are proud to announce the date and venue! The conference will take place on July 9 to 12, 2018 in the unique old physics center in Bad Honnef, Germany.

The location: The small town of Bad Honnef is beautifully located at the Rhine river, about 45 km south of Cologne. It can nicely be reached by train from the airport of Cologne, as well as from Frankfurt airport. The physics center has quite a remarkable history that goes back about 120 years; more information can be found here. Today, it features three state-of-the-art lecture halls and accomodation of about 80 people.

The conference: The main topic of the conference is the work towards a nuclear clock based on Th-229, and we seek to gather all groups around the world that work towards this goal. Further topics include other novel types of optical clocks (e.g. highly charged ions) and applications in geodesy.

The registration: Registration will open later this year, we will send around newsletters and post all information on the nuClock website. Seating capacity limits the number of participants to about 80.

More information will follow. For now, we ask you all to mark your calenders: The meeting will start with a dinner on Sunday, July 8, and end after lunch on Thursday, July 12. To all present nuClock members: This conference will include the internal nuClock yearly meeting 2018.

Looking forward to see you all at the conference!

The picturesque building of the physics center.

The main lecture hall.

Doctor von der Wense

Yesterday, Lars von der Wense of LMU Munich successfully passed his PhD exam, and he did it with summa cum laude distinction! Lars is the first PhD student funded by the nuClock project to finish his thesis. Congratulations!

A snapshot of the PhD exam, the title reads “On the direct detection of the Th-229 nuclear isomer”.

Lars has been supervised by LMU group leader Peter G. Thirolf. The board of examiners also included fellow nuClock group leaders Adriana Palffy from MPIK Heidelberg and Thomas Udem from MPQ Munich. After the 90-minute torture, Lars was rewarded with the traditional doctoral hat prepared by hisPhD colleagues: look at his smile!

Lars wearing his new hat, fully equipped with an alarm clock linked to a Th-229 nucleus!

The celebration continued with a large crowd of family & friends in a nearby restaurant.

Re-evaluation of the Beck et al. data

The currently most accepted value of the isomer energy is 7.8(5) eV, obtained by the so-called “Beck et al. measurement”. In this experiment, researchers from Lawrence Livermore employed a NASA microcalorimeter for a high-resolution gamma measurement of the U-233 decay. Using a clever differencing scheme, they indirectly inferred the energy of the isomer. The final result includes correction terms related to unknown branching ratios.

This measurement is now 10 years old, and in the meantime, no experiment was successful in refining the isomer energy further (there is evidence from the LMU experiment, however, that the isomer energy is between 6.31 and about 18 eV). In particular, two independent experiments at PTB in Germany and UCLA/ALS searched for an optical signal of the isomer using synchrotron radiation to excite nuclei on/in wide bandgap materials, but found no signal. E. V. Tkalya et al. were the first to revisit the Beck et al. measurement and showed that branching ratios markedly different from the ones assumed in the original publication would lead to an isomer energy much larger than 7.8 eV (see their paper here).

Many current experiments have a limited search window, contrained e.g. by the ionization thresholds of Th ions or the transmission window of crystals. Inspired by the mysteriously short lifetime of the Th+ isomer in the LMU experiment, the TU Wien team set out to re-evaluate the original Beck et al. data, aiming to check if the 2007 measurement would be compatible with a much higher isomer energy. Their analysis is now available on the arXiv.

The authors find no major flaw in the original 2007 data analysis. They expand the statistical error of 0.5 eV, which appears to be underestimated, into a two-dimensional contour plot, constructing confidence regions for any given branching ratio. They find that the isomer energy depends only mildly on the branching ratio, in contrast to an earlier analysis by S. L. Zakharov. To give a rough estimate, an isomer energy above 10 eV (below 125 nm) can be excluded at the 95% confidence level.

If you have comments and would like to join the discussion, please share your thoughts with us!

Contour plots of branching ratio b (the probability of the 29-keV state to decay out-of-band onto the ground state) vs isomer energy. The horizontal lines represent various values of b that can be found in the literature. The grey shaded region has been scanned in the UCLA@ALS experiment. The three plots assumes different values of the energy splitting of the 42-keV doublet: (d) 198.22 eV, (e) 198.44 eV, and (f) 198.66 eV.

LMU arXiv paper: feasibility study for electron spectroscopy of the IC electron

A series of recent breakthrough experiments at LMU Munich was able to confirm theoretical and experimental predictions on various properties of the Th-229 isomer. Specifically, the LMU team was able to show (1) that the isomer exists at all, (2) that its energy is somewhere between 6.3 and 18 eV, (3) that the isomer lifetime in Th2+ and Th3+ is longer than a few minutes, (4) that the lifetime in the neutral atom is very short, about 10µs, and (5) that internal conversion (IC) is the dominant decay channel in the neutral atom. The next major topic to address is a refinement of the value of the isomer energy. The original plan of the LMU team was to measure the wavelength of the VUV gamma emitted upon de-excitation (read the corresponding paper here), but as IC seems to be dominant even on large-bandgap surfaces, the LMU team switched gears and now prepares to measure the energy of the IC electron.

In a theoretical feasibility study, LMU PhD student Benedict Seiferle looked into surface effects of the catcher plate and the spectrometer, the attainable resolution, and the expected signal/noise. He concludes that a resolution of 0.1 eV can be reached. Also, he suggests a calibration scheme in which a light source near 160 nm with well-known wavelength is used to, via the photoeffect, knock electrons out of the catcher. This technique could also be used to optimize the spectrometer. Generally, the measured energy spectrum does not depend on the work function of the catcher plate, but only on the retarding voltage applied between the catcher and the spectrometer, and the work function of the spectrometer itself.

The manuscript had been submitted to EPJ D and is now available on the arXiv.