Using 2-photon resonant laser spectroscopy, the PTB team identifies 166 previously uncharted electronic states of the Th1+ ion in the energy range between 7.8 and 9.8 eV. The observed levels can be relevant for the excitation or decay of the 229mTh isomeric nuclear state which lies in this energy range. The high density of electronic levels promises a strongly enhanced electronic bridge excitation of the isomer in 229Th. Read more on arXive.
A VUV frequency comb seems to be a promising candidate for performing high-resolution spectroscopy on the Th-229 nucleus. Such a device up-converts the frequency of a stabilized femtosecond laser from the infrared to the ultraviolet using a process called “high-harmonic generation” (HHG). This process is usually performed in noble gas jets. It requires extreme optical field amplitudes, which usually can only be realized when enhancing the laser power in a passive built-up cavity. The Vienna team has now demonstrated that such a process can be realized more efficiently using a solid-state target than a noble gas jet. The all-solid approach furthermore makes the setup simpler (no gas streaming into vacuum!) and compact. We could show that the solid-state HHG process maintains the original comb structure by performing a beating measurement with a UV CW laser at 266 nm, generously borrowed by the TOPTICA team. Further credits got to Erin Young and Jim Speck from University of California for producing the AlN targets. Read the full article HERE.
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.