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)

Talks:

  • 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)

Posters:

  • 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)

Talks:

  • 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)

Posters:

  • 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.

New publication by the LMU group

The LMU Munich group has published a new paper on the characterization of their experimental set-up. Th-229 recoils (hopefully many of them in the isomeric state) are extracted from a thin U-233 source. The ions are cooled, collimated, and mass-filtered before being gently deposited on a micro-electrode. This part of the set-up has already been published here. Once the Th-229m ion settles on the micro-electrode, it might de-excite into the nuclear and electronic ground state under emission of photons: either the VUV gamma, or secondary photons generated during the de-excitation process. The present paper describes the optical set-up that allows for the detection of these photons: two annular parabolic mirrors image the emission from the micro-electrode onto an MCP, which is monitored by a CCD camera. Given the efficiency of all parts of the experiment and the noise of the detector, a signal-to-noise of 7000:1 can be achieved. The paper can be found on the arXiv preprint server.

New publication by the Vienna group

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.

Visit of Prof. Minkov at MPIK Heidelberg

Prof. Dr. Nikolay Minkov visited the MPIK group in Heidelberg on Nov. 16 and 17. Prof. Minkov works at the Institute for Nuclear Research and Nuclear Energy in Sofia, Bulgaria, and is a specialist in collective models and nuclear deformation. Today, he gave a seminar talk on “Deformations and magnetic moments in nuclear high-K isomeric states”. In addition, there were very fruitful discussions on the nuclear physics part of Th-229. Experts in the theoretical description of nuclear isomers are exactly what we need! Thanks to Nikolay Minkov for visiting!

Nikolay Minov

Prof. Nikolay Minov, visit his webpage for more information.

nuClock library and other resources

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!

Welcome Pavlo Bilous !

The nuClock family welcomes yet another new member: Three weeks ago, Pavlo Bilous started his PhD with Adriana Palffy at MPIK in Heidelberg. Pavlo will boost the theory support of the entire nuClock initiative. He will focus on ab initio calculations of Th-229 electron shell structure. Pavlo is no stranger to the research on thorium: he did his diploma thesis with Leonid Yatsenko at the National Academy of Sciences in Kyiv, Ukraine. During this time, he studied correlations between nuclear and atomic degrees of freedom in Th-229. All the best of luck, Pavlo!

Pavlo Bilous

Pavlo Bilous