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!