arXiv subjects

Christoph E. Düllmann

Publications and source records attributed to Christoph E. Düllmann.

At least 19 recordsLinked to original sources

arXiv2026

Cyclotron-Frequency Stability via Single-Ion Fluorescence in a Penning Trap with a Cryogen-Free Superconducting Magnet

In this work, we investigate the magnetic-field stability of a Penning trap operated with a cryogen-free superconducting magnet through cyclotron-frequency measurements of a single laser-cooled calcium ion using two complementary photon-based detection techniques: a pulsed optical method for determining the three ion eigenfrequencies and Fluorescence-Detected Fourier-Transform Ion-Cyclotron-Resonance (FD-FT-ICR). For the former technique, the data-analysis procedure was revisited, considering it as the dominant contribution to the measurement uncertainty and yielding a long-term magnetic-field drift $(1/B)(dB/dt) = -3.07(32)\times10^{-9}$ $\mathrm{h}^{-1}$. This value is comparable to those reported for high-precision Penning-trap experiments employing liquid-helium-based superconducting magnets. The short-term stability was investigated using the FD-FT-ICR technique, resulting in a minimum relative magnetic-field variation of $δB/B \simeq 5\times10^{-8}$ for averaging times between 30 and 60 s. Furthermore, this technique enables direct cyclotron-frequency determinations on timescales as short as a few seconds, providing access to magnetic-field fluctuations that are generally not resolved in conventional Penning-trap experiments.

physics.ins-det↗

arXiv2026

Multiply charged uranium monoxide as a versatile probe of fundamental physics

Multiply charged actinide molecules provide a unique platform to study fundamental physics and the chemical bond under extreme conditions. Beyond the inherently large relativistic effects associated with a high proton number $Z$, an increased molecular charge can further enhance the electronic sensitivity to symmetry-violating nuclear effects, including nuclear Schiff moments. Experimental investigations of multiply charged actinide molecules are challenging because the high charges severely destabilize chemical bonds, leading to spontaneous Coulomb explosion. We demonstrate a method to systematically generate and detect molecular ions at the edge of chemical stability. By applying high-fluence laser ablation to a depleted uranium metal foil, we produce atomic uranium ions U$^{z+}$ and uranium monoxide cations UO$^{z+}$ with $z = 1$--4. Among them, we observe UO$^{3+}$ and UO$^{4+}$, which exhibit comparatively simple electronic structures and are therefore promising for precision spectroscopy. The experiments are supported by relativistic density functional theory calculations of equilibrium bond lengths, charge distributions, and binding energies of all observed molecules. Calculations of symmetry-violating properties suggest a pronounced sensitivity of UO$^{3+}$ to hadronic $CP$ violation. This approach opens a pathway for high-precision investigations of fundamental symmetries and the exploration of relativistic actinide chemistry in previously inaccessible regimes.

physics.chem-ph↗

arXiv2026

Toward nanophotonic platforms for solid-state $^{229}$Th nuclear clocks

While the $^{229}$Th nuclear isomer has recently been observed and laser-excited, converting optical nuclear manipulation into a chip-scale solid-state frequency standard remains an open challenge. Here, we present a nanophotonic platform to realize an all-solid-state nuclear clock based on the low-energy isomeric transition of $^{229}$Th embedded in high-$Q$ fluoride photonic resonators. By coupling ensembles of thorium nuclei to confined optical modes, we show that resonant field build-up in the cavity can substantially enhance the nuclear excitation rate, enabling optical interrogation at practical laser intensities. We model the nuclei-photon interaction dynamics and outline a technological roadmap toward addressing this challenge, including resonator fabrication in fluoride crystals, thorium implantation, nuclear excitation with integrated lasers, and on-chip detection of vacuum-ultraviolet photons. As an initial proof of concept, we implant a crystalline fluoride whispering-gallery-mode resonator with $^{229}$Th and assess the impact of implantation-induced damage on resonator performance. Our platform leverages recent advances in materials integration and nanophotonics to chart a realistic route toward compact and scalable nuclear frequency standards.

physics.optics↗

arXiv2025

Influence of carbon dioxide and water concentration on terbium thin films produced by Molecular Plating

Terbium and thulium thin films were produced by Molecular Plating under controlled conditions to elucidate a possible influence of water and carbon dioxide present in the plating solution. Platings were made in a glovebox with variable concentration of residual water and \ce{CO2} in a controlled inert atmosphere to study the impact on the quality of the produced thin films and on deposition yields. The morphology of the thin films was analyzed by scanning electron microscopy. The deposition yield was determined by neutron activation analysis at the research reactor TRIGA Mainz. Chemical analysis of the deposited layers was conducted using a combination of infrared, Raman and X-ray photoelectron spectroscopy. The Raman and IR spectra reveal the formation of hydroxides, oxides and carbonates. Water in the plating solution affects the quality of the thin films when its concentration exceeds 1 vol.-\%. The presence of \ce{CO2} leads to an increased carbonate content, which negatively influences the film quality

cond-mat.mtrl-sci↗

arXiv2025

Improved limit on the effective electron neutrino mass with the ECHo-1k experiment

The effective electron neutrino mass can be determined by analyzing the endpoint region of the $^{163}$Ho electron capture spectrum, provided a measurement with high energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in $^{163}$Ho collaboration, ECHo, presents an analysis of the most precise $^{163}$Ho spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of $B=9.1(1.3)\times 10^{-6}$ /eV/pixel/day was achieved allowing for a reliable analysis of the endpoint region. The derived endpoint energy $Q = 2862(4)$ eV is in excellent agreement with the one independently determined via Penning-trap mass spectrometry of $Q=2863.2(6)$ eV [1]. The upper limit of the effective electron neutrino mass is improved by almost a factor 2 compared to the lowest current value [2], reaching $m_{ν_\mathrm{e}} < 15 $ eV/c${^2}$ (90\% credible interval).

hep-ex↗

arXiv2025

A cryogenic Paul trap for probing the nuclear isomeric excited state $^{229\text{m}}$Th$^{3+}$

While laser excitation of the nuclear isomeric transition in $^{229}$Th has been recently achieved for thorium atoms embedded in large-bandgap crystals, laser excitation and characterization of the nuclear transition in trapped $^{229}$Th$^{3+}$ ions has not yet been accomplished. To address these experiments, a cryogenic Paul trap setup has been designed, built, and commissioned at LMU Munich. Here, we present the specifications of the new experimental platform and demonstrate its successful operation, showing the extraction, subsequent ion-guiding, mass-purification, and trapping of $^{229}$Th$^{3+}$ and $^{229\text{m}}$Th$^{3+}$ ions from a newly designed buffer-gas stopping cell as well as of $^{88}$Sr$^{+}$ ions from laser ablation of a solid target. Further, we show sympathetic laser cooling of $^{229\text{(m)}}$Th$^{3+}$ by Doppler-cooled $^{88}$Sr$^{+}$ ions and the formation of mixed-species Coulomb crystals.

physics.atom-ph↗

arXiv2025

Laser fluence-dependent production of molecular thorium ions in different charge states for trapped-ion experiments

Thorium ions and molecules, recognized for their distinctive nuclear and atomic attributes, are central to numerous trapped-ion experiments globally. Our study introduces an effective, compact source of thorium ions produced via laser ablation of microgram-scale, salt-based samples. We thoroughly analyze the variety of ion species and charge states generated at varying laser fluences. Utilizing 10$μ$g of thorium fluoride crystals and laser fluences between $1.00 - 7.00$ J$\cdot$cm$^{-2}$ we produce thorium molecular ions $^{232}$ThF$_x$$^{n+}$ (with $x= 0 - 3$ and charge states up to $n = 3+$), including ThF$^{2+}$ and ThF$^{3+}$. These species are particularly relevant for spectroscopy; ThF$^{3+}$ is valuable for its stable closed-shell configuration, while ThF$^{2+}$, which is isoelectronic to RaF, offers a unique probe for studying nuclear structure and fundamental symmetries due to its simple electronic structure with a single unpaired electron. Density functional theory calculations of the distribution of positive charge in the produced molecular cations and the simplicity of this setup indicate that this method is easily transferable to other actinide systems.

physics.atom-ph↗

arXiv2025

Single-atom-at-a-time adsorption studies of $^{211}$Bi and its precursor $^{211}$Pb on SiO$_{2}$ surfaces

In preparation of gas-phase chemical experiments with moscovium (Mc, element 115), we studied the chemical behavior of the short-lived bismuth radioisotope $^{211}$Bi in helium, argon, and oxygen atmosphere. Internal chromatograms were recorded as a function of various parameters including carrier gas type and flow rate, thus characterizing the novel miniCOMPACT detector array. This aids to optimize the conditions for experiments with superheavy elements. The bismuth progeny of $^{219}$Rn deposited on the SiO$_{2}$ surface of the miniCOMPACT via diffusion-controlled deposition. Bismuth showed the expected high reactivity towards the SiO$_{2}$ surface of the miniCOMPACT. Experiments in argon and oxygen atmosphere showed no measurable differences in the deposition distribution of the activity. The intermediate 36-min $^{211}$Pb is a member of the $^{227}$Ac decay chain, feeding the studied bismuth isotope, was taken into account. To extract thermodynamical data from the results, namely the lower limit of the value of the adsorption enthalpy of Bi on SiO$_{2}$, we performed Monte Carlo simulations, adapted to account for the precursor effect, and compared the experimental results to their output. Simulations were also performed for bismuths heavier homologue, moscovium, using a theoretically predicted value for the adsorption enthalpy of this element on SiO$_{2}$. These suggest moscovium to adsorb in the first part of the miniCOMPACT detection array, in line with recent observations.

nucl-ex↗

arXiv2024

Penning-trap measurement of the $Q$-value of the electron capture in $^{163}\mathrm{Ho}$ for the determination of the electron neutrino mass

The investigation of the absolute scale of the effective neutrino mass remains challenging due to the exclusively weak interaction of neutrinos with all known particles in the standard model of particle physics. Currently, the most precise and least model-dependent upper limit on the electron antineutrino mass is set by the KATRIN experiment from the analysis of the tritium \b{eta}-decay. Another promising approach is the electron capture in $^{163}\mathrm{Ho}$, which is under investigation using microcalorimetry within the ECHo and HOLMES collab orations. An independently measured Q-value of this process is vital for the assessment of systematic uncertainties in the neutrino mass determination. Here, we report a direct, independent determination of this $Q$-value by measuring the free-space cyclotron frequency ratio of highly charged ions of $^{163}\mathrm{Ho}$ and $^{163}\mathrm{Dy}$ in the Penning trap experiment \textsc{Pentatrap}. Combining this ratio with atomic physics calculations of the electronic binding energies yields a $Q$-value of $2863.2(0.6)\,\mathrm{eV}/c^{2}$ - a more than 50-fold improvement over the state-of-the-art. This will enable the determination of the electron neutrino mass on a sub-eV level from the analysis of the electron capture in $^{163}\mathrm{Ho}$.

nucl-ex↗

arXiv2023

Opportunities for Fundamental Physics Research with Radioactive Molecules

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

nucl-ex↗

arXiv2022

Fluorescence calorimetry of an ion crystal

Motivated by the challenge of identifying intruder ions in a cold ion crystal, we investigate calorimetry from emitted fluorescence light. Under continuous Doppler cooling, the ion crystal reaches a temperature equilibrium with a fixed level of fluorescence intensity and any change in the motional energy of the crystal results in a modification of this intensity. We theoretically determine the fluorescence rate of an ion crystal as a function of the temperature, assuming that laser light is scattered along a two-level electronic transition, which couples to the crystal's vibrations via the mechanical effects of light. We analyze how the heat dissipated by collisions of an incoming intruder ion alters the scattering rate. We argue that an energy change by an incoming $^{229}$Th$^{10+}$ ion can be unambiguously detected within 100 $μ$s via illuminating a fraction of a 10$^{3}$ ion crystal. This method enables applications including capture and spectroscopy of charged states of thorium isotopes and investigation of highly charged ions.

physics.atom-ph↗

arXiv2022

Robust Polarization Gradient Cooling of Trapped Ions

We implement three-dimensional polarization gradient cooling of trapped ions. Counter-propagating laser beams near $393\,$nm impinge in lin$\,\perp\,$lin configuration, at a frequency below the S$_{1/2}$ to P$_{3/2}$ resonance in $^{40}$Ca$^+$. We demonstrate mean phonon numbers of $5.4(4)$ at a trap frequency of $2π\times 285\,$kHz and $3.3(4)$ at $2π\times480\,$kHz, in the axial and radial directions, respectively. Our measurements demonstrate that cooling with laser beams detuned to lower frequencies from the resonance is robust against an elevated phonon occupation number, and thus works well for an initial ion motion far out of the Lamb-Dicke regime, for up to four ions, and for a micromotion modulation index $β\leq 0.1$. Still, we find that the spectral impurity of the laser field influences both, cooling rates and cooling limits. Thus, a Fabry-Pérot cavity filter is employed for efficiently suppressing amplified spontaneous emission of the diode laser.

physics.atom-ph↗

arXiv2022

Penning-trap mass measurements of the deuteron and the HD+ molecular ion

The masses of the lightest atomic nuclei and the electron mass are interlinked and are crucial in a wide range of research fields, with their values affecting observables in atomic, molecular and neutrino physics as well as metrology. The most precise values for these fundamental parameters come from Penning-trap mass spectrometry, which achieves relative mass uncertainties in the range of $10^{-11}$. However, redundancy checks using data from different experiments reveal significant inconsistencies in the masses of the proton ($m_p$), the deuteron ($m_d$) and helion ($m_\text{he}$), amounting to $5$ standard deviations for the term $Δ=m_p+m_d-m_{\text{he}}$, which suggests that the uncertainty of these values may have been underestimated. Here we present results from absolute mass measurements of the deuteron and the ${HD}^+$ molecular ion against $^{12}C$ as a mass reference. Our value for the deuteron $m_d=2.013\,553\,212\,535 (17)$u supersedes the precision of the literature value by a factor of $2.4$ and deviates from this by $4.8$ standard deviations. With a relative uncertainty of $8$ parts per trillion (ppt) this is the most precise mass value measured directly in atomic mass units. Furthermore, the measurement of the ${HD}^+$ molecular ion, $m({HD}^+)=3.021\,378\,241\,561\,(61)$u, not only allows for a rigorous consistency check of our measurements of the masses of the deuteron (this work) and proton, but also establishes an additional link for the masses of tritium and helium-3 to the atomic mass unit. Combined with a recent measurement of the deuteron-to-proton mass ratio the uncertainty of the reference value of $m_p$ can be reduced by a factor of three. This is a post-peer-review, pre-copyedit version of an article published in Nature. The final authenticated version is available online at https://doi.org/10.1038/s41586-020-2628-7

physics.atom-ph↗

arXiv2020

Alpha spectrometric characterization of thin $^{233}$U sources for $^{229\text{(m)}}$Th production

Four different techniques were applied for the production of $^{233}$U alpha recoil ion sources, providing $^{229}$Th ions. They were compared with respect to a minimum energy spread of the $^{229}$Th recoil ions, using the emitted alpha particles as an indicator. The techniques of Molecular Plating, Drop-on-Demand inkjet printing, chelation from dilute nitric acid solution on chemically functionalized silicon surfaces, and self-adsorption on passivated titanium surfaces were used. All fabricated sources were characterized by using alpha spectrometry, radiographic imaging, and scanning electron microscopy. A direct validation for the estimated recoil ion rate was obtained by collecting $^{228}$Th recoil ions from $^{232}$U recoil ion sources prepared by self-adsorption and Molecular Plating. The chelation and the self-adsorption based approaches appear most promising for the preparation of recoil ion sources delivering monochromatic recoil ions.

physics.ins-det↗

arXiv2020

Specific Heat of Holmium in Gold and Silver at Low Temperatures

The specific heat of dilute alloys of holmium in gold and in silver plays a major role in the optimization of low temperature microcalorimeters with enclosed $^{163}\textrm{Ho}$, such as the ones developed for the neutrino mass experiment ECHo. We investigate alloys with atomic concentrations of $x_\textrm{Ho}=0.01\,\% - 4\,\%$ at temperatures between $10\,\textrm{mK}$ and $800\,\textrm{mK}$. Due to the large total angular momentum $J=8$ and nuclear spin $I=7/2$ of $\textrm{Ho}^{3+}$ ions, the specific heat of $\underline{\textrm{Au}}\textrm{:Ho}$ and $\underline{\textrm{Ag}}\textrm{:Ho}$ depends on the detailed interplay of various interactions, including contributions from the localized 4f electrons and nuclear contributions via hyperfine splitting. This makes it difficult to accurately determine the specific heat of these materials numerically. Instead, we measure their specific heat by using three experimental set-ups optimized for different concentration and temperature ranges. The results from measurements on six holmium alloys demonstrate that the specific heat of these materials is dominated by a large Schottky anomaly with its maximum at $T\approx 250\,\textrm{mK}$, which we attribute to hyperfine splitting and crystal field interactions. RKKY and dipole-dipole interactions between the holmium atoms cause additional, concentration-dependent effects. With regard to ECHo, we conclude that for typical operating temperatures of $T\leq 20\,\textrm{mK}$, silver holmium alloys with $x_\textrm{Ho}\gtrsim 1\,\%$ are suited best.

cond-mat.mtrl-sci↗

arXiv2020

Measurement of the $^{229}$Th isomer energy with a magnetic micro-calorimeter

We present a measurement of the low-energy (0--60$\,$keV) $γ$ ray spectrum produced in the $α$-decay of $^{233}$U using a dedicated cryogenic magnetic micro-calorimeter. The energy resolution of $\sim$$10\,$eV, together with exceptional gain linearity, allow us to measure the energy of the low-lying isomeric state in $^{229}$Th using four complementary evaluation schemes. The most accurate scheme determines the $^{229}$Th isomer energy to be $8.10(17)\,$eV, corresponding to 153.1(37)$\,$nm, superseding in precision previous values based on $γ$ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be $b_{29}=9.3(6)\%$ and $b_{42}=0.3(3)\%$.

nucl-ex↗

arXiv2020

Development of a recoil ion source providing slow Th ions including $^{229(m)}$Th in a broad charge state distribution

Ions of the isomer $^{229m}$Th are a topic of high interest for the construction of a "nuclear clock" and in the field of fundamental physics for testing symmetries of nature. They can be efficiently captured in Paul traps which are ideal for performing high precision quantum logic spectroscopy. Trapping and identification of long-lived $^{232}$Th$^{+}$ ions from a laser ablation source was already demonstrated by the TACTICa collaboration on Trapping And Cooling of Thorium Ions with Calcium. The $^{229m}$Th is most easily accessible as $α$-decay daughter of the decay of $^{233}$U. We report on the development of a source for slow Th ions, including $^{229(m)}$Th for the TACTICa experiment. The $^{229(m)}$Th source is currently under construction and comprises a $^{233}$U monolayer, from which $^{229(m)}$Th ions recoil. These are decelerated in an electric field. Conservation of the full initial charge state distribution of the $^{229(m)}$Th recoil ions is one of the unique features of this source. We present ion-flight simulations for our adopted layout and give a final design. This source will provide Th ions in their original charge state at energies suitable for capture in a linear Paul trap for spectroscopy investigations.

physics.atom-ph↗

arXiv2019

Production of highly charged ions of rare species by laser-induced desorption inside an electron beam ion trap

This paper reports on the development and testing of a novel, highly efficient technique for the injection of very rare species into electron beam ion traps (EBITs) for the production of highly charged ions (HCI). It relies on in-trap laser-induced desorption of atoms from a sample brought very close to the electron beam resulting in a very high capture efficiency in the EBIT. We have demonstrated a steady production of HCI of the stable isotope $^{165}\mathrm{Ho}$ from samples of only $10^{12}$ atoms ($\sim$ 300 pg) in charge states up to 45+. HCI of these species can be subsequently extracted for use in other experiments or stored in the trapping volume of the EBIT for spectroscopic measurements. The high efficiency of this technique expands the range of rare isotope HCIs available for high-precision nuclear mass and spectroscopic measurements. A first application of this technique is the production of HCI of the synthetic radioisotope $^{163}\mathrm{Ho}$ for a high-precision measurement of the $Q_{\mathrm{EC}}$-value of the electron capture in $^{163}\mathrm{Ho}$ within the Electron Capture in Holmium experiment (ECHo collaboration) ultimately leading to a measurement of the electron neutrino mass with an uncertainty on the sub-eV level.

physics.atom-ph↗