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.
Recently, a Russian theory group took a close look at the electronic structure of the Th atoms adsorbed to the crystal surface, and found a number of pathways that would allow the isomeric state to decay back into the ground state without emission of a UV photon. This finding, available online from the (arXiv preprint server) could explain the null measurement.
Speaking of unsuccessful attempts to pin down the isomer transition, one should also mention beautiful recent work of Eric Hudson’s group at UCLA. Here, the US researchers used highly specialized UV-transparent crystals, heavily doped with Th-229 atoms, as the target for a synchrotron beam. The team had access to the Advanced Light Source (ALS) in Berkeley, CA. This facility generates a strong flux of photons, however the wavelength could only be scanned over about half the expected wavelength region. No signal was found, which might lead to one of the following three conclusions: (1) the transition wavelength is not within the scanned region of 140 to 170 nm, (2) the isomer lifetime is much smaller or much larger than expected, or (3) emission of the UV photon is suppressed by the demon of internal conversion. The work can be found on the arXiv and has been published with PRL.