Optical pumping to the Th-229 isomeric state demonstrated

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.