The radio isotope 229-Thorium shows a remarkable and unique property: it possesses an extremely low-energy excited (isomer) state of the nucleus (3+/2 [631] in Nilsson classification) which is expected around 7.6 ± 0.5 eV. Electromagnetic radiation corresponding to this energy is around 160 ± 10 nm wavelength, which is in the ultraviolet (UV) regime. It may hence be possible to create an excited state of an atomic nucleus using (laser) light! It is the aim of this project to find and characterize this low-energy nuclear transition and make it accessible for fundamental investigations and applications.

Figure 1. Level scheme of the lowest-energy isomer state in 229-Thorium

The existence of the 7.6 eV nuclear transition in 229-Thorium is undisputed and experimentally evidenced by high-energy gamma spectroscopy of the theoretically predicted higher excited states of the nucleus. The exact transition wavelength yet remains unknown. So does the lifetime of the excited state, which from Mössbauer spectroscopy of similar systems is estimated to be around 1000 seconds.

The high transition energy and the (expected) narrow line width make the nuclear transition a promising candidate for a novel frequency standard. One can expect efficient shielding of the transition by the electron shell against external magnetic or electric fields. Thorium ions can be embedded in UV transparent crystals (e.g. CaF2, LiCaAF6) and hence realize an optical solid-state “nuclear atomic clock”. The complicated and bulky vacuum system currently required by atomic clocks could be replaced by a single crystal at room temperature doped with 229-Thorium atoms. Effects of the internal crystal fields on the transition (line shifts and broadenings) will have to be investigated.

The low-energy nuclear transition frequency is based on the interplay of the Coulomb and the strong interaction inside the nucleus. Comparing with different time standards based on (electromagnetic) hyperfine transitions will allow to measure temporal variations of fundamental constants in tabletop experiments. Because of the large energy scales inherent to nuclear interactions, the sensitivity to such variations can be increased by a least a factor of 1000.

Figure 2. Crystal structure of CaF as a possible host for 229-Thorium