While several groups make progress on determining the exact energy of the Th-229 isomer, the uncertainty is still too large for a direct optical excitation with a narrow-band source to be successful. Most approaches use the U-233 alpha-decay as a means of populating the isomer with a 2% probability. This alpha decay is however a violent process, transferring >80 keV of recoil energy to the “new-born” Th-229 ion, which makes further manipulation a formidable task. A Japanese consortium, with support by TU Wien and associated Okayama University and TU Wien, has now succeeded in populating the the isomer, starting from the Th-229 ground state, transferring only a negligible amount of recoil energy. The team resonantly excites to the second nuclear level at 29 keV using the SPring-8 synchrotron facility, which then predominantly decays to the Th-229 isomer, realizing highly efficient “x-ray pumping” into the isomer. The energy of the 2nd nuclear level is measured with extreme accuracy (0.07 eV), also the decay branching ratio from the 2nd nuclear state back to ground- and isomeric state respectively, is measured experimentally for the first time. This two numbers allow a re-interpretation of the “historic” gamma measurements on this system which have been performed for over 40 years. The preprint of the article can be found here.
Brenden, Wen-Te, and Adriana of MPIK in Heidelberg have released a new paper investigating collective effects that could occur in Thorium doped crystals when excited by narrow-band coherent pulses. Here’s the link to the paper. A variety of schemes are discussed with the goal of creating unique signatures of excitation which can be used to determine the detuning of the exciting laser pulses from 229Th nuclear transition energy. Other details such as multi pulsed excitation, pulse shape, phase shifting, static magnetic fields and quantization axis are discussed to give a comprehensive understanding of the possibilities when tackling such a project.
A personal note from the nuClock coordinator: This work fulfills the last formal deliverable for EU reporting, rejoice!
Marianna Safronova and colleagues from the Univ. of Delaware and from the Kurchatov Institute in St. Petersburg have performed new atomic structure calculations for Th+and Th2+that are used to relate measured spectroscopic isotope shifts to differences of the nuclear charge radii. Such calculations are notoriously difficult for these thorium ions because of strong configuration mixing of the electrons. Combined with experimental data on isotope shifts in Th+from 227Th to 232Th, measured at KfK Karlsruhe in 1989, and the recent results on isotope shifts of Th2+obtained by PTB and LMU within nuclock, they provide a more reliable picture of the thorium nuclear charge radii, including an improved value for the radius change between 229Th and the229mTh isomer: Excitation to the isomeric state increases the charge radius by less than 0.02%. The work is published in Physical Review Letters.
Starting with October 22., Tomas Sikorsky is enforcing the TU Wien Thorium team. Tomas has worked on NMR during his masters and on ion collisions with ultra cold gasses during his PhD in the team of Roee Ozeri at Weizmann. In Vienna, he will establish detection of the Thorium isomer via nuclear quadrupole resonance spectroscopy (NQRS). Welcome Tomas!
Every year, the German Physics Society (DPG) offers a PhD thesis prize for each of its sections. This year, Lars von der Wense of LMU Munich was awarded the thesis prize in the section “Matter and Cosmos”. Congratulations to Lars, his supervisor Peter G. Thirolf, and the whole team at LMU for this prestigious recognition!
This year’s European Frequency and Time Forum (EFTF) is already the 32nd edition of this conference series and will take place in Torino, Italy, during the upcoming week. This conference is one of the global gathering of researchers involved with the measurement and distribution of time and frequencies; the conference webpage can be found here. Each year, a European Time and Frequency Award is given to an outstanding researcher to recognize his or her work. This year’s EFTF Laureate is Ekkehard Peik (PTB), one of the driving forces behind the nuClock project. He will receive the award “for seminal contributions to single-ion optical frequency standards and high-precision spectroscopy thereby establishing most stringent limits on possible variations of fundamental constants”. Congratulations to Ekkehard for being this year’s EFTF Laureate!
The Th-229 nuclear isomer has been around for more than 40 years already, but two of its main properties, namely its energy and lifetime, are known only with very large error margins. While there is no experimental value for the bare isomer lifetime at all, there is at least some consensus on the isomer energy (somewhere between 6.3 and 10 eV). In their recent study (now available on the arXiv preprint server here), the Russian group at MEPhI suggests an energy of 7.1 eV and a lifetime of about half an hour for the bare isomer. These values are in agreement with all recent experiments, which is very good news. And it’s the first time since the Beck et al. measurement (more than 10 years ago !) that a research team dares to put forward a value of the isomer energy. Let’s hope that an independent experiment using a different approach will soon be able to confirm this value. Congratulations to the Russian team for their work!
The TU Wien group and the Metrology Light Source (MLS) in Berlin (a part of PTB) have joined forces to directly excite the Thorium isomer in the VUV. In a measurement campaign about a year ago, Th-229 doped crystals were illuminated by tunable undulator radiation at the MLS facility. A wavelength region between 124 and 240 nm was probed (5.2 to 10 eV) at illumination & detection times between 30 and 600 seconds. The result of this study has now been made available on the arXiv preprint server (find the publication here).
In short, massive photoluminescence masked any possible gamma emission of the isomer. Three different types of photocathodes were used (Cs-Te, Cs-I, diamond), where only the Cs-I version was capable of supressing the photoluminescence at longer wavelengths to a degree that allowed researchers to draw any conclusions on the energy and lifetime of the isomer. Assuming radiative decay as the only decay channel, an isomer lifetime between 0.2 and 1.1 seconds can be excluded for an isomer energy between 7.5 and 10 eV (the region of highest sensitivity of the Cs-I detector). This study complements earlier work in the group of Eric Hudson at UCLA, where a parameter region of similar energy, but longer lifetime was excluded. Also, the results of this study are in agreement with the work at LMU.
Although not successful in this campaign, the results give direct input to the design and protocol of the next round of measurements.
Researchers at LMU Munich have put together a new review paper on the history of work on the Th-229 isomer, its current status, as well as potential implementations of a nuclear clock. The paper also investigates the properties of other candidates (aside from Th-229) to be used for a nuclear clock. It was assembled in connection with a talk given by Lars von der Wense at last year’s 175th anniversary of the Mendeleev All-Russia Research Institute of Metrology (VNIIM) in St. Petersburg, and can be downloaded here.