Precise energy determination of the isomeric ground-state transition in 229mTh: Advancing on the route towards a nuclear clock

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In the recently terminated DFG project, which is hereby applied for renewal, our group achieved a breakthrough result in a research field which is intensely studied worldwide since 40 years: we achieved the first direct identification of the decay (and thus the existence) of the energetically lowest excited states amongst all known atomic nuclei, namely the isomeric first excited state in 229-thorium. This state attracts widespread attention due to its potential to serve as a highly precise nuclear frequency standard (nuclear clock). However, in order to realize a nuclear clock it needs an improved knowledge of the transition energy (presently assumed to be 7.8(5) eV) to allow for the construction of an optimized laser for an optical manipulation of the transition. Our present measurements were designed for the identification of the transition, not for its energy determination. This will be the central objective of the present project renewal proposal. Our detection of the decay of 229mTh was based on the collection of doubly or triply charged Th ions on a detector surface with subsequent registration of the decay products. These were electrons from the internal conversion (IC) decay channel, which in case of neutral thorium (which has to be expected here due to the charge neutralization on the detector surface) is energetically allowed and has to be expected to dominate. Thus a precise measurement of the transition energy will have to focus on a precise determination of the kinetic energy of low-energy IC electrons. For this purpose we will set up a high-resolution magnetic bottle electron spectrometer. Here the IC electrons will first be efficiently collected and focused in a suitable magnetic field configuration and then guided to an electron detector. On their way they will have to pass several potential meshed with retarding potentials, which will lead to a sharp cut-off edge in the electron spectrum at the kinetic energy of the IC decay electrons. This will allow to determine the isomeric transition energy to better than 50 meV, one order of magnitude more precise than the present value. This is sufficient to enable the construction of a laser system that will optically excite the nuclear transition to the isomer. The required neutralization of the thorium ions will be induced in a first project phase on a metallic collection surface. However, in order to exclude any potentially distorting surface effects, in parallel a contact-free neutralization will be developed, using a Cs atomic-vapor based charge exchange cell. The final energy determination will be conducted with this configuration. Characterization and calibration of the spectrometer will be performed with a helium gas discharge lamp. This project will allow our group to keep or even extend the present lead in the quest for the 229mTh nuclear clock transition, with the final goal to lay the foundations for the realization of an ultra-precise nuclear frequency standard.