Physikalisch-Technische Bundesanstalt

Gamma spectroscopy has shown that the nucleus of ²²⁹Th possesses an isomeric state at the unusually low excitation energy of 7.6 ± 0.5 eV. Light of the corresponding transition wavelength in the range of 160 nm can be produced by frequency upconversion of tunable lasers and of optical frequency combs so that laser spectroscopy methods can be applied to a nuclear system for the first time. The long-lived excited nuclear state interacts with the electron shell only weakly, so that very high resolution can be expected even in solid samples of high particle density. This makes the ²²⁹Th system attractive for fundamental investigations as well as for the application as an optical frequency standard.

The objective of this project is to demonstrate high-resolution laser spectroscopy of the nuclear transition of ²²⁹Th using ions stored in a trap. Besides Th⁺, also Th³⁺ is of interest because its simple electronic level system is suitable for laser cooling. The excitation of the nucleus can be detected with high efficiency through a double-resonance method by probing the changes of the hyperfine structure of electronic energy levels.