Study of 229mTh: towards an ultra-precise all-optical nuclear clock
Towards optical access to the lowest nuclear excitation in 229Th
Photograph of the 229Th isomer generator and detection system.
For a long the first excited nuclear level energy of 229Th was reported as (3.5±1.0) eV, being the lowest known first excited level in nuclear physics. In a recent experiment the transition energy was remeasured, resulting in an important change of the energy to E_gamma = (7.6±0.5) eV . The corresponding wavelength of the transition lies in the deep UV at lambda = (163±11) nm, which means that it still can be focused with lenses and addressed by optical methods. The estimated lifetime of the level is tau = 1 - 5 hours, corresponding to a natural line width of only Delta E = 10-19 eV and an extremely small ratio Delta E/E = 10-20. Thus the 7.6 eV ground state transition carries the potential of an ultra-precise all-optical nuclear clock. Moreover, it has been shown  that a precise measurement of the variation of the transition energy with time could result in an enhanced sensitivity for an identification of potential variations of fundamental constants like the fine structure constant or the strong interaction parameter by several orders of magnitude. One has to approach this excited level of 229Th in a stepwise procedure: first the UV fluorescence signal around 7.6 eV has to be confirmed and the present accuracy of the transition energy has to be improved to below about 1 nm. In a further step 229Th3+ ions offer a suitable level scheme for laser cooling as a prerequisite for aiming at an optical control of the 7.6 eV transition with a suitable laser then to be developed for the precise transition energy. At the MLL in Garching an experimental setup has been commissioned that allows to separate the population of the excited state in 229mTh via alpha-decay from 233U in a buffer gas cell from the detection of the UV deexcitation signal after transporting the 229mTh isomers out of the gas cell and collecting them on a needle tip followed by a suitable UV focussing optics and detection system (see Fig. 1).
1.) B.R. Beck et al., Energy Splitting of the Ground-State Doublets in the Nucleus 229Th, Phys. Rev. Lett. 98 (2007) 142501
2.) V.W. Flambaum, Enhanced Effect of Temporal Variation of the Fine Structure Constant and the Strong Interaction in 229Th, Phys. Rev. Lett. 97 (2006) 092502