Vacuum-ultraviolet spectroscopy of the radiative decay of the low-energy isomer in 229Th

The radioisotope 229Th features a nuclear isomer 229mTh with an exceptionally low excitation energy of ≈ 8 eV and a favourable coupling to the environment, making it a candidate for a next generation of optical clocks allowing to study fundamental physics. While first indirect experimental evidence for the existence of such a nuclear state dates from 46 years ago, the proof of existence has been delivered only recently by observing the isomer’s internal electron conversion decay. This discovery triggered a series of successful measurements of several properties, amongst others, its energy. In spite of recent progress, the difficulties to observe the isomer’s radiative decay remains a dark spot of this research field. The development towards a "nuclear clock" is further hindered by a too large uncertainty on the isomer energy.

In order to overcome limitations of previous experiments and to increase the population of the isomer while easing at the same time background contributions, a novel approach is used to populate the isomeric state in radioactive decay. It is based on the β-decay of 229Ac and uses radioactive ion beams provided by the ISOLDE facility at CERN implanted into large-bandgap crystals. Starting with this concept, a custom vacuum ultraviolet spectroscopy setup for the efficient detection of photons from the decay of the low-energy isomer has been developed in this thesis.

An experimental campaign led to the first direct detection of the radiative decay of this low-energy isomer. By performing vacuum ultraviolet spectroscopy of 229mTh embedded into large-band gap crystals, the photon wavelength of its decay was measured as 148.71(42) nm, corresponding to an excitation energy of 8.338(24) eV. The half-life of 229Th embedded in MgF2 was determined. The improved uncertainty of the isomer’s energy by a factor of seven eases the search for direct laser excitation of the atomic nucleus while the observation of the radiative decay in a large-bandgap crystal and an estimate of its half-life have important consequences for the design of a future nuclear clock.