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 arXiv version can be found here. 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!

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.