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

TL;DR

This paper discusses the experimental efforts to directly measure the UV fluorescence of the lowest nuclear excitation in 229Th, aiming to refine its energy value for applications in optical clocks and fundamental physics.

Contribution

It reports on experimental activities to identify the UV fluorescence of 229Th's nuclear transition and improve the energy measurement accuracy, enabling future laser control and fundamental studies.

Findings

Corrected the nuclear excitation energy to 7.6(5) eV

Aims to detect UV fluorescence of the nuclear transition

Lays groundwork for optical clock development

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 $\tau=$ 3-5 hours, leading to an extremely sharp relative linewidth of Delta E/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. Moreover, in the literature speculations about a potentially enhanced sensitivity of the ground-state transition of $^{229m}$Th for eventual time-dependent variations of fundamental constants (e.g. fine structure constant alpha) can be found. 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.