Laser spectroscopic characterization of the nuclear clock isomer 229mTh

Johannes Thielking, Maxim V. Okhapkin, Przemyslaw Glowacki, David M. Meier, Lars von der Wense, Benedict Seiferle, Christoph E. Düllmann, Peter G. Thirolf, Ekkehard Peik


The isotope 229Th is the only nucleus known to possess an excited state 229mTh in the energy range of a few electron volts, a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than common nuclear excitation energies. A number of applications of this unique nuclear system, which is accessible by optical methods, have been proposed. Most promising among them appears a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of 229mTh2+, yielding values of fundamental nuclear properties, namely the magnetic dipole and electric quadrupole moments as well as the nuclear charge radius. After the recent direct detection of this long-searched-for isomer, our results now provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection, an important step towards the development of a nuclear clock.

Laser-induced electronic bridge for characterization of the 229mTh→ 229gTh nuclear transition with a tunable optical laser

Pavlo V. Bilous, Ekkehard Peik, Adriana Pálffy


An alternative method to determine the excitation energy of the 229mTh isomer via the laser-induced electronic bridge is investigated theoretically. In the presence of an optical or ultra-violet laser at energies that fulfill a two-photon resonance condition, the excited nuclear state can decay by transfering its energy to the electronic shell. A bound electron is then promoted to an excited state by absorption of a laser photon and simultaneous de-excitation of the nucleus. We present calculated rates for the laser-induced electronic bridge process and discuss the experimental requirements for the corresponding setup. Our results show that depending on the actual value of the nuclear transition energy, the rate can be very high, with an enhancement factor compared to the radiative nuclear decay of up to 10^8.

Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 μJ pulse energy and 300 W average power

Johannes Weitenberg, Andreas Vernaleken, Jan Schulte, Akira Ozawa, Thomas Sartorius, Vladimir Pervak, Hans-Dieter Hoffmann, Thomas Udem, Peter Russbüldt, Theodor W. Hänsch


We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 μJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced  compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.

Nubis et Nuclei: A study on noise and precision

Kerstin Ergenzinger, Thorsten Schumm, Simon Stellmer


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


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


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.

Optomechanically induced transparency of x-rays via optical control

Wen-Te Liao and Adriana Pálffy


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.

Internal conversion from excited electronic states of Th-229 ions

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


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


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


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.

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