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.

While there is little doubt about the existence of the Th-229 isomeric state, a direct measurement of its energy and lifetime are still pending. The current best value of the isomer wavelength is 159(10) nm, where the error stated here is purely statistical. The uncertainty needs to improved by at least one or two orders of magnitude before is makes sense to commence direct laser spectroscopy. So far, experiments employing synchrotron radiation for excitation of the isomer have not succeeded to detect the photonic de-excitation of the isomer. While each experiment might suffer from individual challenges, is seems that non-radiative decay of the isomer might dominate over the radiative branch whenever the thorium atom is adsorbed onto (or doped into) a medium.

The Vienna group now put forward a new approach to measure the isomer emission with a spectrometer. The method of isomer population itself has been used dozens of times before: U-233 undergoes alpha decay into Th-229, and, with a 2%-probability, into the isomeric state. The technological advancement is to go from surface-implated Th ions to Th ions doped into crystals. Leaping from 2-D to 3-D increases the flux of isomer gammas by two orders of magnitude! The new approach is experimentally much easier to implement and requires no pre-knowledge or “guessing” of the isomer lifetime. In a practical experiment, the signal is expected to be increased by more like 3 orders of magnitude.

A number of careful experiments were required to show that this new approach is indeed feasible. At first, it was shown that uranium can be doped into CaF₂ at concentrations of 1000 ppm. Second, the transparency of ²³³U:CaF₂ crystals in the VUV was shown to be sufficiently good to allow gammas to leave the crystal. The alpha decay of U-233 leads to scintillation of the crystal, and it could be shown that this luminescence appears only in a spectral region that is well-separated from the expected isomer transition.

The wavelength region around 160 nm is, however, covered by Cherenkov radiation. This radiation does not originate directly from the alpha decay, but from beta decays in the chain of Th-229. Luckily, Th-229 has a half-life (8000 years) much larger than a typical experiment. Sadly, commonly available U-233 is contaminated with U-232, which decays into Th-228 and further down the entire chain on timescales comparable to the experiment. It is the beta decays in the unavoidable Th-228 contamination that form the Cherenkov background. With a bit of theory input, the absolute amplitude of Cherenkov emission can be calculated numerically. The only input parameter required is the amount of U-232 contamination and the date of the last removement of thorium ingrowth.

Modelling a future experiment with reasonable parameters, such as a U-232 contamination of 10 ppm, a 3-month period since the last purification, and a standard commercial VUV spectrometer, it is found that even if 99% of the isomers undergo non-radiative de-excitation, a clear signal can be detected within a few days of measurement. This value jumps to many months if the non-radiative decay comes in at 99.9%.

Demonstrating the feasibility of this approach required contributions from experiment, theory, and radiochemistry: a truly multi-disciplinary effort! The preprint, backed by a good deal of supplemental material, can be retrieved from the arXiv pre-print server.

It’s been precisely 3 months since the nuClock website went online! Every day, the website is currently visited by 30 people on average. Close to 500 visitors have found our site already and clicked on our pages more than 3000 times.

Today, we are launching a new category, called “resources”. As a service to the entire Thorium research community, we have established a repository for information and data that we believe to be useful. It might be especially helpful for all of the newly established research groups around the world that are currently joining the Thorium research activities. We are starting with a library of the 100+ publications that we consider most relevant for our field of research. There is also a set of links to fabulous places on the internet where one can find nuclear and atomic physics data. In addition, we have started with a list of Thorium research groups around the world (yet not affiliated with nuClock).

We seek to constantly expand our data and knowledge “resources” section. At the same time, it will remain a carefully curated selection of truly essential information (and not a dump for all the stuff that we ran acoss). Please send us material that you find could be useful for the community!

This week sees a symposium on frequency standards and metrology taking place in Potsdam, Germany. The symposium is held roughly every five years, always at a different locations. This year’s edition is organized by PTB in Braunschweig, namely by the chair, Fritz Riehle, and his team. The main topics of the symposium are microwave and optical clocks, as well as the comparison between clocks over ever increasing distances.

nuClock will be represented by Ekkehard Peik of PTB, delivering a talk entitled “Search for the low-energy isomer in 229Th and the nuclear optical clock”. As a second PTB contribution, Maksim Okhapkin will present a poster “Towards electronic bridge excitation of the low-energy nuclear isomer in Th-229”. Simon Stellmer of TU Wien will be substituted by Lars von der Wense and Benedict Seiferle of LMU Munich: they will present a joint TU Wien / LMU poster covering recent (and exciting !) work.

All in all, there will be some 50 talks and 110 posters spread over the entire week. The conference begins on Monday Oct. 12th with a keynote by Nobel laureat Dave Wineland. The conference website can be found here.