Knowing the wavelength of the Th-229 isomer transition is utterly important, as it determines which schemes for a nuclear clock will work, and which ones will not. This value, however, is not yet known precisely, and it is one of the first aims of nuClock to determine the transition wavelength.
To add some fun to the search, we are introducing TIB, the Thorium Isomer Bet. Here’s how it works: Make a guess of what the transition wavelength λ is, and quantify how certain you are about your bet (Δλ). Towards the end of the nuClock project, we will determine the actual value λ0 and its uncertainty Δλ0, based on latest measurements. Whoever came closest to the actual value will be invited to the final nuClock meeting, held in June 2019 in Vienna, and receive a mighty jéroboam (3-liter) bottle of champagne!
To join the game, send your values of λ and Δλ (in nanometers) to Simon. Everyone (expert or not, within or outside of nuClock) is invited, bets are accepted until the end of this year.
For the experts: Each bet will be modelled as a laser of wavelength λ, linewidth Δλ, and unit intensity, and we will calculate how many Th nuclei each laser can excite, using the Lorentzian-type expression for off-resonant excitation. Largest number N wins!
Ekkehard Peik and Maxim Okhapkin published a new review article on the unique low-energy two-level system in the nucleus of Th-229. This up-to-date review paper describes all past and present approaches to find the isomer transition and outlines possible schemes for a future optical clock. The article “Nuclear clocks based on resonant excitation of gamma-transitions” has been published in a special issue entitled “The measurement of time” in the French series of books Comptes Rendus Physique, edited by Christophe Salomon.
Yet another attempt to find the Th isomer transition! The PTB group in Braunschweig, headed by Ekkehard Peik, teamed up with the MLS group in Berlin to perform optical spectroscopy of surface-adsorbed Th-229. In a first step, PTB researcher Atsushi Yamaguchi undertook an excursion into the voodoo of wet chemistry to find the optimum recipe to persuade the precious Th-229 atoms to firmly stick to the surface of a VUV-transparent MgF2 disk. In a second step, this probe was placed into the synchrotron radiation beam provided by the Metrology Light Source (MLS), a large PTB facility in Berlin. The wavelength of the radiation was changed in steps, and in between the steps, UV-sensitive photomultipliers (PMTs) recorded the fluorescence of the sample. Unfortunately, no resonance was found. The work has been published in the New Journal of Physics. Read more
Simon Stellmer and co-workers have published a long paper that summarizes a whole series of experiments performed within the last year. The studies are concerned with the detailed characterization of the photoluminescence, radioluminescence, and thermoluminescence properties of Th-doped CaF2 crystals. Such crystals will be used as a platform for direct optical spectroscopy of the isomer transition. There are, however, considerable worries that the glowing of the crystal itself, as it is subjected to radioactivity and strong UV light, might be much stronger than the faint sought-after nuclear signal. The studies show that the worries are indeed justified, as the crystals emit characteristic luminescence in the UV range. Luckily, the emission wavelength is sufficiently far away from the expected nuclear transition, such that a great deal of crystal luminescence can be filtered away spectrally. The researchers also characterized the decay time of the luminescence and its dependence on temperature. The findings allowed them to develop strategies to further reduce the luminescence background. All in all, this experimental work proofs that CaF2 crystals are a viable platform for optical spectroscopy. The paper is available on the arXiv and has been published with Nature Scientific Reports.
nuClock is on its way! Finally, after more than a year of gathering ideas, writing the proposal, and setting up the contract, we were able to set sails and embark on the new project. Impossible to say into which direction the thorium research will move during the next four years. Stay with us, it’s for sure going to be exciting!