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

Abstract

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 frominfluences of external fields in the framework of the linear Zeeman andquadratic Stark effect, rendering 229mTh a candidate for a reference of anoptical 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-dependentvariations 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 theground-state transition energy as a prerequisite for a laser-based opticalcontrol of this nuclear excited state, allowing to build a bridge between atomic and nuclear physics and open new perspectives for metrological as wellas fundamental studies.