PUBLICATIONS

The reduced magnetic dipole and electric quadrupole transition probabilities for the radiative decay of the 229Th 7.8 eV isomer to the ground state are predicted within a detailed nuclear-structure model approach. We show that the presence and decay of this isomer can only be accounted for by the Coriolis mixing emerging from a remarkably fine interplay between the coherent quadrupole-octupole motion of the nuclear core and the single-nucleon motion within a reflection-asymmetric deformed potential. We find that the magnetic dipole transition probability which determines the radiative lifetime of the isomer is considerably smaller than presently estimated. The so-far disregarded electric quadrupole component may have non-negligible contributions to the internal conversion channel. These findings support new directions in the experimental search of the 229Th transition frequency for the development of a future nuclear frequency standard.

Future photonic quantum networks will require interfaces between different photon frequency regimes. Here we envisage for the first time an optomechanical system that bridges optical photons and x-rays with the aim to control the latter and exploit their unique properties regarding large momentum transfer, focusability, penetration, robustness and detection efficiency. The x-ray-optical interface system comprises of an optomechanical cavity and a movable microlever interacting with both an optical laser and with x-rays via resonant nuclear scattering. We develop a theoretical model for this system and show that x-ray absorption spectra of nuclei can be tuned optomechanically. In particular, our theoretical simulations predict optomechanically induced transparency of x-rays.

The process of internal conversion from excited electronic states is investigated theoretically for the case of the vacuum-ultraviolet nuclear transition of 229Th. Due to the very low transition energy, the 229Th nucleus offers the unique possibility to open the otherwise forbidden internal conversion nuclear decay channel for thorium ions via optical laser excitation of the electronic shell. We show that this feature can be exploited to investigate the isomeric state properties via observation of internal conversion from excited electronic configurations of Th+ and Th2+ ions. A possible experimental realization of the proposed scenario at the nuclear laser spectroscopy facility IGISOL in Jyväskylä, Finland is discussed.

The presently accepted value of the energy splitting of the Th-229 ground-state doublet has been obtained on the basis of undirect gamma spectroscopy measurements by Beck et al., Phys. Rev. Lett. 98, 142501 (2007). Since then, a number of experiments set out to measure the isomer energy directly, however none of them resulted in an observation of the transition. Here we perform an analysis to identify the parameter space of isomer energy and branching ratio that is consistent with the Beck et al. experiment.

The first excited isomeric state of Th-229 possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today’s laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground state (internal conversion, γ decay, and bound internal conversion), whose strengths depend on the charge state of Th-229m. We report on the measurement of the internal-conversion decay half-life of neutral Th-229m. A half-life of 7±1 μs has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of about 10,000 s of the photonic decay channel, gives further support for an internal conversion coefficient of about 10^9, thus constraining the strength of a radiative branch in the presence of internal conversion.

We propose a simple approach to measure the energy of the few-eV isomeric state in Th-229. To this end, U-233 nuclei are doped into VUV-transparent crystals, where they undergo alpha decay into Th-229, and, with a probability of 2 %, populate the isomeric state. These Th-229m nuclei may decay into the nuclear ground state under emission of the sought-after VUV gamma, whose wavelength can be determined with a spectrometer.

Based on measurements of the optical transmission of U:CaF2 crystals in the VUV range, we expect a signal at least 2 orders of magnitude larger compared to current schemes using surface-implantation of recoil nuclei. The signal background is dominated by Cherenkov radiation induced by beta decays of the thorium decay chain. We estimate that, even if the isomer undergoes radiative de-excitation with a probability of only 0.1 %, the VUV gamma can be detected within a reasonable measurement time.

We investigate a potential candidate for a future optical clock: the nucleus of the isotope ²²⁹Th. Over the past 40 years of research, various experiments have found evidence for the existence of an isomeric state at an energy of a few eV. So far, neither the energy nor the lifetime of the isomer have been determined directly. As the scene is not yet prepared for direct laser excitation, other means of populating the isomer need to be explored. We investigate three different approaches, all of which rely on CaF₂ crystals doped with ²²⁹Th or ²³³U. Various kinds of crystal luminescence are discussed in detail.

Today’s most precise time and frequency measurements are performed with optical atomic clocks. However, it has been proposed that they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of an atomic shell transition. There is only one known nuclear state that could serve as a nuclear clock using currently available technology, namely, the isomeric first excited state of 229Th (denoted 229mTh). Here we report the direct detection of this nuclear state, which is further confirmation of the existence of the isomer and lays the foundation for precise studies of its decay parameters. On the basis of this direct detection, the isomeric energy is constrained to between 6.3 and 18.3 electronvolts, and the half-life is found to be longer than 60 seconds for 229mTh2+. More precise determinations appear to be within reach, and would pave the way to the development of a nuclear frequency standard.

With an expected energy of 7.6(5) eV, ²²⁹Th possesses the lowest excited nuclear state in the landscape of all presently known nuclei. The energy corresponds to a wavelength of about 160 nm and would conceptually allow for an optical laser excitation of a nuclear transition. We report on a VUV optical detection system that was designed for the direct detection of the isomeric ground-state transition of ²²⁹Th. ^229(m)Th ions originating from a ²³³U α-recoil source are collected on a micro electrode that is placed in the focus of an annular parabolic mirror. The latter is used to parallelize the UV fluorescence that may emerge from the isomeric ground-state transition of ²²⁹Th. The parallelized light is then focused by a second annular parabolic mirror onto a CsI-coated position-sensitive MCP detector behind the mirror exit. To achieve a high signal-to-background ratio, a small spot size on the MCP detector needs to be achieved. Besides extensive ray-tracing simulations of the optical setup, we present a procedure for its alignment, as well as test measurements using a D₂ lamp, where a focal-spot size of ≈100 μm has been achieved. Assuming a purely photonic decay, a signal-to-background ratio of ≈7000:1 could be achieved.

In the whole landscape of atomic nuclei, ²²⁹Th is currently the only known nucleus which could allow for the development of a nuclear-based frequency standard, as it possesses an isomeric state of just 7.6 eV energy above the ground state. The 3+ charge state is of special importance in this context, as Th³⁺ allows for a simple laser-cooling scheme. Here we emphasize the direct extraction of triply-charged ²²⁹Th from a buffer-gas stopping cell. This finding will not only simplify any future approach of ²²⁹Th ion cooling, but is also used for thorium-beam purification and in this way provides a powerful tool for the direct identification of the ²²⁹Th isomer to ground state nuclear transition.