Breakthrough: The first optical spectroscopy of Th-229m ions

So far, all experiments that characterized the Th-229 nuclear isomer employed nuclear physics techniques: gamma spectroscopy, alpha spectroscopy, detection of electrons, coincidence schemes, and the like. For the nuclear optical clock, however, technology out of the quantum optics toolbox will be requires, such as lasers, optical detection, and precision spectroscopy. A recent experiment by the PTB, LMU, and GSI groups now made a huge step into this direction: they performed the first laser spectroscopy of electronic states in Th-229m ions.

The experimental realization was truely a team effort: at first, the hyperfine structure of Th-229 in its nuclear ground state was measured at the thorium ion trap at PTB. Then, all the lasers and required optics were brought to LMU Munich to measure the combined Th-229 + Th-229m in the LMU ion trap. A U-233 recoil source was used to produce the Th-229m nuclei in the isomeric state. The combined spectrum of Th-229 and Th-229m clearly showed additional peaks that were not present in the pure Th-229 measurements. The hyperfine structure of two different electronic levels was investigated, and the number of additional peaks was sufficient to determine the A and B parameters for these two levels in Th-229m. A comparison with the Th-229 nucleus then allowed the authors to calculate the magnetic moment of the Th-229m nucleus. The value of -0.37(6) µ_N is about five times larger than the previously accepted value derived  from the Nilsson model. In addition, the quadrupole moment of the isomer was determined to be Q=1.74(6) eb. From this value, one can infer that the geometric shape of the nuclear charge distribution of the isomer is very similar to the one of the nuclear ground state. The difference in the mean-square radii of the ground and isomeric states is calculated as 0.012(2) fm^2. With these values, we have a very clear image of what the isomer looks like.

Following the first direct detection of the isomeric state and the determination of the isomer lifetime unter internal conversion decay, this work is the third major breakthrough within the nuClock project. The corresponding publications can now be retrieved from the arXiv preprint server here.

Combined hyperfine structure of the transition at 1164 nm, connecting the 20711 and 29300 electronic states in Th2+. The cloud of ions, containing 2% ions in the isomeric state, are used for spectroscopy. The four small peaks, labelled with quantum numbers and highlighted in cyan color, belong to the isomeric state Th-229m. These peaks are not present in a pure sample of Th-229 with all ions in the nuclear ground state.

Theory paper on electron bridge excitation of the isomer

The direct excitation of the nuclear isomeric state is hindered by (1) the technological challenge of building a narrow-linewidth, coherent laser source, and (2) the small coupling of the nucleus to the laser field. These two limitations can be overcome by using an electron bridge process, in which at least part of the required excitation energy is provided by the excitation of a suitable long-lived electronic state. This approach is followed by the ion trap group of E. Peik at PTB. In recent theory work, the group of A. Palffy at MPIK Heidelberg investigated a suitable scheme using Th3+ ions. The manuscript is now available here on the arXiv.

The best of two worlds: A new proposal for optical spectroscopy of the isomer transition

A handful of experiments have tried optical excitation of the isomer already, unfortunately without success. All of these experiments searched for delayed fluorescence in the optical domain as the smoking gun of an excitation of the isomer. The main obstancle in these experiments can be summarized as follows: The transition linewidth is teeny-weeny small, probably about 0.001 Hz, but the linewidth of excitation sources is very broad, about 100,000,000,000,000 Hz. So it’s very unlikely to excite the nucleus. The small excitation probability can be offset by using many many nuclei, say 10^15 nuclei. Such large numbers of atoms need to be cast into some solid form, either as a metal, a dopant into a some sort of host material, or a layer attached to some underlying material. But once the isomer is confined in a solid, it tends to undergo internal conversion: it will de-excite by emitting an electron rather than a photon. This process might explain why previous experiments, which searched for an optical signal, were not successful.

Now, Lars von der Wense (LMU Munich group) proposes to use the best of two worlds: optical excitation via lasers, detection via electrons. There do exist pulsed lasers with sufficiently small linewidth and sufficiently large power to make this approach feasible. In addition, the detection of the isomer via spectroscopy of the IC electron is also well established in Munich: there shall be nothing in the way of this experiment.

This proposal has now been accepted by Phys. Rev. Lett. (find the abstract here) and will be published within the next couple of weeks. The list of co-authors includes researchers from 4 out of the 8 nuClock partners: Half the consortium was involved in this proposal.

Congratulations to Lars and the team!

Simon Stellmer receives ERC Starting Grant

Simon Stellmer, nuClock researcher on the Vienna team, has received an ERC Starting Grant. The title of his project reads “Ultracold mercury for a measurement of the EDM”. Within this project, he will address one of the most fundamental questions in all of physics: Why does the Universe contain matter? Shortly after the Big Bang, many billion years ago, equal amounts of matter and antimatter were formed. These two types of matter, however, destroy themselves when they come into contact. This process is called annihilation, and naively, one would conclude that matter and antimatter annihilated completely some time after the Big Bang. Quite obviously, this conclusion is at odds with observations.

So there must be a fundamental asymmetry between matter and antimatter: an underlying mechanism that favors matter over antimatter. This mechanism ensured that, as matter and antimatter annihilated, a small excess portion of matter survived: this is the matter that forms our Universe today. The details of this mechanism, however, are still a mystery.

The asymmetry between matter and antimatter is connected to a phenomenon called CP-violation, which, in short, states that going backwards in time is not the same as going forward in time. This phenomenon shows up as a tiny tiny ellipticity of fundamental particles (electrons, neutrons and the like): the charge distribution of these particles is not perfectly spherical, but a little deformed. This deformation can be measured in high-precision measurements. A number of such experiments were carried out already, but none of them was sensitive enough to detect these small deformations. Dr. Stellmer aims to improve the sensitivity of these experiments by taking them into the quantum world: previous experiments were performed with room-temperature gases of mercury atoms. He will now cool these gases to temperatures one millionth of a degree above absolute zero: this is where quantum phenomena emerge, which Dr. Stellmer seeks to exploit for improving the measurement performance.

ERC Grants are among the most prestigious prizes awarded to researchers in Europe. The project will be funded with 2 M€ by the European Union.

Dr. Stellmer received a prestigious ERC grant.

The press release is available in here.

Reading material for a nice summer evening

Sitting in your deck chair with nothing to read? We have a solution for you!

A few weeks ago, Francisco Ponce of Lawrence Livermore / UC Davis finished his PhD thesis on the topic “High Accuracy Measurement of the Nuclear Decay of U-235m and Search for the Nuclear Decay of Th-229m”. In his studies, he searched for the IC electron in the de-excitation of the Th isomer, but was not sensitive to timescales in the µs range. Although eventually not successful, the PhD thesis still makes a nice reading. The thesis can be found here.

And while we’re at it, we would like to draw your attention to another study from Jason Burke’s LLNL group, which looked into the distribution of charge states of the Th-229 recoils following the alpha decay of U-233: this work can be found here.