Wider research context / theoretical framework
Thorium-229 presents a spectacularly low-energy nuclear excited state around8 eV. This so-called isomer is within reach of advanced laser systems, opening up the possibility to manipulate nuclear states with lasers. A multitude of exciting applications has been proposed for this system, ranging from a robust high-precision optical nuclear clock to the search for variations of the fundamental interaction constants of physics.
Hypotheses/research questions /objectives
In the last years, impressive progress has been made in pinning down the energy of the Th-229 isomer. Still, the uncertainty is on the 5% level; a huge search range for laser spectroscopy. In fact, no direct optical manipulation of the isomer has yet been demonstrated: neither the excitation from the ground state nor the radiative decay of the isomer has been observed. These are the main aims of the REThorIC project.
Approach/methods
The coupling of the M1 nuclear low-energy transition to (laser) light is weak. To compensate, we follow a solid-state approach, which provides 10 orders of magnitudes more interrogated nuclei than possible in trapped ions. Th-229 is doped into a VUV-transparent CaF crystal matrix, with densities up to 1e18 per cubic cm. Three methods for controlled excitation of the isomer will be used:
- X-ray pumping by resonant excitation of the 2nd nuclear state at 29 keV using the Okayama SPring-8 facility
- direct resonant excitation from the ground state using a strong broadband VUV e-beam source in Vienna
- direct resonant excitation from the ground state using and a 4-wave-mixing laser system at PTB Braunschweig
To detect the notoriously weak isomer signal and measure its energy, we will employ broadband detection with strong temporal and spectral background filtering as well as a unique high-NA VUV spectrometer.
Level of originality / innovation
Demonstrating optical manipulation of the Th-229 isomer would represent a breakthrough that the community is awaiting for several decades. It appears possible now due to the new powerful excitation sources and methods available to the applicants, combined with the massively increased number of interrogated nuclei within the crystal approach.
Primary researchers involved
The project will be carried out jointly be the team of K. Yoshimura and N Sasao from Okayama University and the team of T. Schumm from TU WIEN. Okayama brings in access and approved beam time at the SPring-8 synchrotron facility and the experience in broadband VUV detection. Vienna provides Th-229-doped crystals, direct excitation sources, and expertise and equipment in VUV spectroscopy. Both teams look back on >5 years of fruitful collaboration yielding several joint publications (e.g. Nature 573, 238 (2019)). Together, they could recently demonstrate optical pumping into the Th-229 isomeric state; transferring this result to VUV crystals is a central component of the REThorIC project.
