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Continuous-wave laser absorption spectroscopy of the Thorium-229 nucleus
Authors:
I. Morawetz,
T. Riebner,
L. Toscani De Col,
F. Schneider,
N. Sempelmann,
F. Schaden,
M. Bartokos,
G. A. Kazakov,
S. Lahs,
K. Beeks,
B. Gerstenecker,
A. Grüneis,
M. Pimon,
T. Schumm,
V. Lal,
G. Zitzer,
V. Petrov,
J. Tiedau,
M. V. Okhapkin,
E. Peik
Abstract:
A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the na…
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A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave narrow-bandwidth laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The VUV laser source is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride crystal and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient 0.1 V/$Å^2$, to be compared to gradients in the range of 100 V/$Å^2$ observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are nearly independent of the lattice spacing.
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Submitted 16 June, 2026; v1 submitted 17 April, 2026;
originally announced April 2026.
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Laser Mössbauer spectroscopy of ^{229}Th
Authors:
Takahiro Hiraki,
Takahiko Masuda,
Sayuri Takatori,
Fabian Schaden,
Michael Bartokos,
Kjeld Beeks,
Yuta Fukunaga,
Andreas Grüneis,
Ming Guan,
Georgy Kazakov,
Thomas LaGrange,
Adrian Leitner,
Ira Morawetz,
Ryoichiro Ogake,
Koichi Okai,
Martin Pimon,
Martin Pressler,
Thomas Riebner,
Noboru Sasao,
Felix Schneider,
Thorsten Schumm,
Kotaro Shimizu,
Luca Toscani de Col,
Tomas Sikorsky,
Akihiro Yoshimi
, et al. (1 additional authors not shown)
Abstract:
Mössbauer spectroscopy is widely used in biochemistry, geology, and solid-state physics to obtain structural information on materials. Here, we extend this technique into the optical range using a vacuum ultraviolet laser to probe the low-energy nuclear transitions of thorium-229, doped in calcium fluoride crystals. We discover four distinct doping sites for the thorium ions, determine the charact…
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Mössbauer spectroscopy is widely used in biochemistry, geology, and solid-state physics to obtain structural information on materials. Here, we extend this technique into the optical range using a vacuum ultraviolet laser to probe the low-energy nuclear transitions of thorium-229, doped in calcium fluoride crystals. We discover four distinct doping sites for the thorium ions, determine the characteristic electric field gradients emerging in the interaction with the host crystal, and identify the microscopic structure of the two dominant configurations. Site-selective laser excitation allows to study the isomeric state lifetime and laser-induced quenching for all sites. This laser-based Mössbauer spectroscopy provides a powerful probe of the nuclear environment, yielding foundational data for designing future solid-state nuclear clocks.
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Submitted 23 August, 2025;
originally announced September 2025.
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X-ray-induced quenching of the $^{229}$Th clock isomer in CaF$_2$
Authors:
Ming Guan,
Michael Bartokos,
Kjeld Beeks,
Hiroyuki Fujimoto,
Yuta Fukunaga,
Hiromitsu Haba,
Takahiro Hiraki,
Yoshitaka Kasamatsu,
Shinji Kitao,
Adrian Leitner,
Takahiko Masuda,
Nobumoto Nagasawa,
Koichi Okai,
Ryoichiro Ogake,
Martin Pimon,
Martin Pressler,
Noboru Sasao,
Fabian Schaden,
Thorsten Schumm,
Makoto Seto,
Yudai Shigekawa,
Kotaro Shimizu,
Tomas Sikorsky,
Kenji Tamasaku,
Sayuri Takatori
, et al. (5 additional authors not shown)
Abstract:
Thorium-229 has the lowest nuclear-excited state (an isomer state) at approximately 8.356 eV, making it excitable with tabletop vacuum-ultraviolet lasers. Despite the recent success of laser excitation, the isomer quenching inside the solid-state environment remains unresolved. In this letter, we present experiments investigating X-ray-induced isomer quenching in the CaF$_2$ host, focusing on the…
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Thorium-229 has the lowest nuclear-excited state (an isomer state) at approximately 8.356 eV, making it excitable with tabletop vacuum-ultraviolet lasers. Despite the recent success of laser excitation, the isomer quenching inside the solid-state environment remains unresolved. In this letter, we present experiments investigating X-ray-induced isomer quenching in the CaF$_2$ host, focusing on the effects of X-ray flux and temperature on the lifetime and yield of the isomer state. Our studies reveal a correlation between isomer production, isomer lifetime during irradiation, and post-irradiation afterglow of the target crystal across different temperatures, highlighting a strong relationship between isomer quenching and color-center dynamics. We developed a model to interpret the isomer quenching and the crystal's luminescence.
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Submitted 6 August, 2025; v1 submitted 5 May, 2025;
originally announced May 2025.
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Laser-Induced Quenching of the Th-229 Nuclear Clock Isomer in Calcium Fluoride
Authors:
F. Schaden,
T. Riebner,
I. Morawetz,
L. Toscani De Col,
G. A. Kazakov,
K. Beeks,
T. Sikorsky,
T. Schumm,
K. Zhang,
V. Lal,
G. Zitzer,
J. Tiedau,
M. V. Okhapkin,
E. Peik
Abstract:
The 10-minute radiative lifetime of the first excited $^{229}$Th$^{4+}$ nuclear state in ionic crystals provides narrow spectroscopic linewidths, enabling the realization of a solid-state nuclear clock. Due to the 4+ noble gas configuration, electronic readout or state initialization schemes known from atomic clocks are inaccessible. This elongates the interrogation cycle, which will deteriorate t…
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The 10-minute radiative lifetime of the first excited $^{229}$Th$^{4+}$ nuclear state in ionic crystals provides narrow spectroscopic linewidths, enabling the realization of a solid-state nuclear clock. Due to the 4+ noble gas configuration, electronic readout or state initialization schemes known from atomic clocks are inaccessible. This elongates the interrogation cycle, which will deteriorate the clock performance. To address this limitation we demonstrate laser-induced quenching (LIQ) as a method of depumping the $^{229}$Th isomer population in CaF$_2$. We provide experimental evidence for LIQ at different wavelengths (148 - 420 nm) and temperatures (100 - 350 K), achieving a threefold reduction in the isomer lifetime with 20 mW of laser power.
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Submitted 16 December, 2024;
originally announced December 2024.
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An Embedding Cluster Approach for Accurate Electronic Structure Calculations of (229)Th:CaF2
Authors:
Kamil Nalikowski,
Valera Veryazov,
Kjeld Beeks,
Thorsten Schumm,
Marek Krosnicki
Abstract:
Building on recent advances of the embedded cluster approach combined with multiconfigurational theory, this work investigates the electronic states in thorium-doped CaF2 crystals. Th:CaF2 is currently establishing as a promising material for solid-state nuclear clocks, which utilize the laser-accessible isomeric state in thorium-229. By comparing simulated absorption spectra of a library of defec…
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Building on recent advances of the embedded cluster approach combined with multiconfigurational theory, this work investigates the electronic states in thorium-doped CaF2 crystals. Th:CaF2 is currently establishing as a promising material for solid-state nuclear clocks, which utilize the laser-accessible isomeric state in thorium-229. By comparing simulated absorption spectra of a library of defect configurations with experimental data, we demonstrate the impact of fluorine vacancies and calcium vacancies on the Th:CaF2 electronic structure. Our results indicate that fluorine-deficient sites can introduce local electronic states within the band gap, resonant with the isomer energy, potentially contributing to non-radiative decay or quenching of the Th-229 isomer. We also explore the potential of electron-nuclear bridge mechanisms to enhance nuclear excitation or de-excitation, offering a pathway for more efficient control over the nuclear clock. This study provides key insights for optimizing the crystal environment for nuclear metrology applications and opens new avenues for further experimental and theoretical exploration of thorium-doped ionic crystals.
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Submitted 11 October, 2024; v1 submitted 30 September, 2024;
originally announced October 2024.
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Optical Transmission Enhancement of Ionic Crystals via Superionic Fluoride Transfer: Growing VUV-Transparent Radioactive Crystals
Authors:
Kjeld Beeks,
Tomas Sikorsky,
Fabian Schaden,
Martin Pressler,
Felix Schneider,
Björn N. Koch,
Thomas Pronebner,
David Werban,
Niyusha Hosseini,
Georgy Kazakov,
Jan Welch,
Johannes H. Sterba,
Florian Kraus,
Thorsten Schumm
Abstract:
The 8 eV first nuclear excited state in $^{229}$Th is a candidate for implementing an nuclear clock. Doping $^{229}$Th into ionic crystals such as CaF$_2$ is expected to suppress non-radiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of $^{229}$Th prohibits the growth of high-quality single crystals with high $^{229}$T…
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The 8 eV first nuclear excited state in $^{229}$Th is a candidate for implementing an nuclear clock. Doping $^{229}$Th into ionic crystals such as CaF$_2$ is expected to suppress non-radiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of $^{229}$Th prohibits the growth of high-quality single crystals with high $^{229}$Th concentration; radiolysis causes fluoride loss, increasing absorption at 8 eV. We overcome this roadblock by annealing $^{229}$Th doped CaF$_2$ at 1250$\unicode{x2103}$ in CF$_4$. The technique presented here allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants which can be used in deep UV optics, laser crystals, scintillators, and nuclear clocks.
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Submitted 29 February, 2024; v1 submitted 21 December, 2023;
originally announced December 2023.
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Growth and characterization of thorium-doped calcium fluoride single crystals
Authors:
Kjeld Beeks,
Tomas Sikorsky,
Veronika Rosecker,
Martin Pressler,
Fabian Schaden,
David Werban,
Niyusha Hosseini,
Lukas Rudischer,
Felix Schneider,
Patrick Berwian,
Jochen Friedrich,
Dieter Hainz,
Jan Welch,
Johannes H. Sterba,
Georgy Kazakov,
Thorsten Schumm
Abstract:
We have grown $^{232}$Th:CaF$_2$ and $^{229}$Th:CaF$_2$ single crystals for investigations on the VUV laser-accessible first nuclear excited state of $^{229}$Th. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of $^{229}$Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the verti…
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We have grown $^{232}$Th:CaF$_2$ and $^{229}$Th:CaF$_2$ single crystals for investigations on the VUV laser-accessible first nuclear excited state of $^{229}$Th. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of $^{229}$Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the vertical gradient freeze method on 3.2 mm diameter seed single crystals with a 2 mm drilled pocket, filled with a co-precipitated CaF$_2$:ThF$_4$:PbF$_2$ powder in order to grow single crystals. Concentrations of $4\cdot10^{19}$ cm$^{-3}$ have been realized with $^{232}$Th with good ($>$10%) VUV transmission. However, the intrinsic radioactivity of $^{229}$Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, limiting the $^{229}$Th concentration to $<5\cdot10^{17}$ cm$^{-3}$.
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Submitted 10 November, 2022;
originally announced November 2022.
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Nuclear excitation of the $^{229}$Th isomer via defect states in doped crystals
Authors:
Brenden S. Nickerson,
Martin Pimon,
Pavlo V. Bilous,
Johannes Gugler,
Kjeld Beeks,
Tomas Sikorsky,
Peter Mohn,
Thorsten Schumm,
Adriana Pálffy
Abstract:
When Th nuclei are doped in CaF$_2$ crystals, a set of electronic defect states appears in the crystal bandgap which would otherwise provide complete transparency to vacuum-ultraviolet radiation. The coupling of these defect states to the 8 eV $^{229m}$Th nuclear isomer in the CaF$_2$ crystal is investigated theoretically. We show that although previously viewed as a nuisance, the defect states pr…
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When Th nuclei are doped in CaF$_2$ crystals, a set of electronic defect states appears in the crystal bandgap which would otherwise provide complete transparency to vacuum-ultraviolet radiation. The coupling of these defect states to the 8 eV $^{229m}$Th nuclear isomer in the CaF$_2$ crystal is investigated theoretically. We show that although previously viewed as a nuisance, the defect states provide a starting point for nuclear excitation via electronic bridge mechanisms involving stimulated emission or absorption using an optical laser. The rates of these processes are at least two orders of magnitude larger than direct photoexcitation of the isomeric state using available light sources. The nuclear isomer population can also undergo quenching when triggered by the reverse mechanism, leading to a fast and controlled decay via the electronic shell. These findings are relevant for a possible solid-state nuclear clock based on the $^{229m}$Th isomeric transition.
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Submitted 20 July, 2020; v1 submitted 15 April, 2020;
originally announced April 2020.