Towards a direct transition energy measurement of the lowest nuclear excitation in Th-229

The isomeric first excited state of the isotope 229Th exhibits the lowest nuclear excitation energy in the whole landscape of known atomic nuclei. For a long time this energy was reported in the literature as 3.5(5) eV, however, a new experiment corrected this energy to 7.6(5) eV, corresponding to a UV transition wavelength of 163(11) nm. The expected isomeric lifetime is τ = 3-5 hours, leading to an extremely sharp relative linewidth of ΔE/E ≈ 10−20, 5-6 orders of magnitude smaller than typical atomic relative linewidths. For an adequately chosen electronic state, the frequency of the nuclear ground-state transition will be independent from influences of external fields in the framework of the linear Zeeman and quadratic Stark effect, rendering 229mTh a candidate for a reference of an optical clock with very high accuracy [1]. Moreover, in the literature speculations about a potentially enhanced sensitivity of the ground-state transition of 229mTh for eventual time-dependent variations of fundamental constants (e.g. fine structure constant α) can be found [3,4]. We report on our experimental activities that aim at a direct identification of the UV fluorescence of the ground-state transition energy of 229mTh. A further goal is to improve the accuracy of the ground-state transition energy as a prerequisite for a laser-based optical control of this nuclear excited state, allowing to build a bridge between atomic and nuclear physics and open new perspectives for metrological as well as fundamental studies.status: publishe