PUBLICATIONS

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.

Measuring the Th-229 nuclear isomer transition with U-233 doped crystals

Simon Stellmer, Matthias Schreitl, Georgy Kazakov, Johannes H. Sterba, and Thorsten Schumm

11/24/2015

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.

An Optomechanical Interface Bridging X-Ray and Optical Photons

Wen-Te Liao and Adriana Pálffy

08/27/2015

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.

Nuclear clocks based on resonant excitation of gamma-transitions

Ekkehard Peik and Maxim Okhapkin

06/05/2015

We review the ideas and concepts for a clock that is based on a radiative transition in the nucleus rather than in the electron shell. This type of clock offers advantages like an insensitivity against field-induced systematic frequency shifts and the opportunity to obtain high stability from interrogating many nuclei in the solid state. Experimental work concentrates on the low-energy (about 8 eV) isomeric transition in 229Th. We review the status of the experiments that aim at a direct optical observation of this transition and outline the plans for high-resolution laser spectroscopy experiments.

Radioluminescence and photoluminescence of Th:CaF2 crystals

Simon Stellmer, Matthias Schreitl, and Thorsten Schumm

06/05/2015

We study thorium-doped CaF2 crystals as a possible platform for optical spectroscopy of the Th-229 nuclear isomer transition. We anticipate two major sources of background signal that might cover the nuclear spectroscopy signal: VUV-photoluminescence, caused by the probe light, and radioluminescence, caused by the radioactive decay of Th-229 and its daughters. We find a rich photoluminescence spectrum at wavelengths above 260 nm, and radioluminescence emission above 220 nm. This is very promising, as fluorescence originating from the isomer transition, predicted at a wavelength shorter than 200 nm, could be filtered spectrally from the crystal luminescence. Furthermore, we investigate the temperature-dependent decay time of the luminescence, as well as thermoluminescence properties. Our findings allow for an immediate optimization of spectroscopy protocols for both the initial search for the nuclear transition using synchrotron radiation, as well as future optical clock operation with narrow-linewidth lasers.

Experimental search for the low-energy nuclear transition in Th-229 with undulator radiation

A. Yamaguchi, M. Kolbe, H. Kaser, T. Reichel, A. Gottwald, and E. Peik

05/28/2015

To search for the lowest energy nuclear isomeric transition in Th-229 in solid samples, a novel adsorption technique which prepares Th-229 atoms on a surface of CaF2 is developed. Adsorbed Th-229 is exposed to highly intensive undulator radiation in the wavelength range between 130 and 320 nm, which includes the indirectly measured nuclear resonance wavelength 160(10) nm. After the excitation, fluorescence from the sample is detected with a VUV sensitive photomultiplier tube. No clear signal relating to the nuclear transition is observed and possible reasons are discussed.

Observation of an unexpected negative isotope shift in 229Th+ and its theoretical explanation

M.V. Okhapkin, D.M. Meier, E. Peik, M.S. Safronova, M.G. Kozlov, and S.G. Porsev

05/21/2015

We have measured the hyperfine structure and isotope shifts of the 402.0 nm and 399.6 nm resonance lines in 229Th+. These transitions could provide pathways towards the 229Th isomeric nuclear state excitation. An unexpected negative isotope shift relative to 232Th+ is observed for the 399.6 nm line, indicating a strong Coulomb coupling of the excited state to the nucleus. We have developed a new all-order approach to the isotope shift calculations that is generally applicable to heavy atoms and ions with several valence electrons. The theoretical calculations provide an explanation for the negative isotope shift of the 399.6 nm transition and yield a corrected classification of the excited state. The calculated isotope shifts are in good agreement with experimental values.