PUBLICATIONS

Nubis et Nuclei: A study on noise and precision

Kerstin Ergenzinger, Thorsten Schumm, Simon Stellmer

06/20/2017

This study sets out to explore the perception of noise, as well as the recovery of meaning or information that it might contain, in arts, science, and daily life. It is realized as an installation based on an arrangement of nitinol drums that create a sonic environment evolving in time and space. The nitinol drums are driven by digital random noise. The observer is free to explore the sonic environment, and will discover regions in time and space with a “meaningful” signal. This discovery of a clear signal in a noisy background holds strong analogies to the scientific search for a nuclear resonance performed in the nuClock project.

Feasibility Study of Internal Conversion Electron Spectroscopy of Th-229m

Benedict Seiferle, Lars von der Wense, Peter G. Thirolf

05/25/2017

With an expected energy of 7.8(5) eV, the isomeric first excited state in Th-229 exhibits the lowest excitation energy of all known nuclei. Until today, a value for the excitation energy has been inferred only by indirect measurements. In this paper, we propose to use the internal conversion decay channel as a probe for the ground-state transition energy. MatLab-based Monte Carlo simulations have been performed to obtain an estimate of the expected statistics and to test the feasibility of the experiment. From the simulations we conclude, that with the presented methods an energy determination with a precision of better than 0.1 eV is possible.

Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in Th-229

Nikolay Minkov, Adriana Pálffy

05/23/2017

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.

Internal conversion from excited electronic states of Th-229 ions

Pavlo V. Bilous, Georgy A. Kazakov, Iain D. Moore, Thorsten Schumm, Adriana Pálffy

03/13/2017

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.

Re-evaluation of the Beck et al. data to constrain the energy of the Th-229 isomer

Georgy A. Kazakov, Thorsten Schumm, Simon Stellmer

02/02/2017

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.

Lifetime Measurement of the Th-229 Nuclear Isomer

Benedict Seiferle, Lars von der Wense, and Peter G. Thirolf

01/26/2017

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.

Direct detection of the 229Th nuclear clock transition

Lars von der Wense, Benedict Seiferle, Mustapha Laatiaoui, Jürgen B. Neumayr, Hans-Jörg Maier, Hans-Friedrich Wirth, Christoph Mokry, Jörg Runke, Klaus Eberhardt, Christoph E. Düllmann, Norbert G. Trautmann & Peter G. Thirolf

05/05/2016

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.

A VUV detection system for the direct photonic identification of the first excited isomeric state of Th-229

Benedict Seiferle, Lars von der Wense, Mustapha Laatiaoui, and Peter G. Thirolf

03/15/2016

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.

The extraction of 229Th3+ from a buffer-gas stopping cell

L.v.d. Wense, B. Seiferle, M. Laatiaoui, and P.G. Thirolf

02/05/2016

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.

A VUV detection system for the direct photonic identification of the first excited isomeric state of Th-229

Benedict Seiferle, Lars von der Wense, Mustapha Laatiaoui, and Peter G. Thirolf

11/24/2015

With an expected energy of 7.6(5) eV, Th-229 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 Th-229. Th-229m ions originating from a U-233 alpha 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 Th-229. 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 m has been achieved. Assuming a purely photonic decay, a signal-to-background ratio of about 7000:1 could be achieved.

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