Logo of the Physikalisch-Technische Bundesanstalt
Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Optical nuclear spectroscopy of 229Th

Linear ion trap for storage of Thorium ions

Linear ion trap for storage of Thorium ions

Gamma spectroscopy has shown that the nucleus of 229Th possesses an isomeric state at the unusually low excitation energy of 7.8 ± 0.5 eV. Light of the corresponding transition wavelength in the range of 160 nm can be produced by frequency upconversion of tunable lasers so that laser spectroscopy methods can be applied to a nuclear system for the first time. For suitably chosen states of the electron shell, the long-lived excited nuclear state and the nuclear transition frequency can be largely decoupled from external perturbations. This makes the 229Th system attractive for fundamental studies of connections between atomic and nuclear physics as well as for the application as a highly precise optical clock.

The main objective of our project is to demonstrate high-resolution laser spectroscopy of the nuclear transition of 229Th using ions stored in a trap. In Th+ and Th2+ the coupling between nucleus and electron shell is important and can be used for studies of the nuclear structure.  The ion  Th3+ is of interest because its simple electronic level system is suitable for direct 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.

We cooperate within the European project nuClock with groups form nuclear physics and quantum optics.

A review on the concepts and approaches towards a 229Th nuclear clock is presented in: E. Peik, M. Okhapkin: Nuclear clocks based on resonant excitation of gamma-transitions [pdf]. C. R. Physique 15, 516-523 (2015)

Further reading