Charge Radii of

Nuclear charge radii of ^{55,56}Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus ^{56}Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the ^{56}Ni core is supposed to be much softer than the ^{48}Ca core. The very low magnetic moment µ(^{55}Ni)=-1.108(20) µ_{N} indicates the impact of M1 excitations between spin-orbit partners across the N,Z=28 shell gaps. Our charge-radii results are compared to ab initio and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.

Charge Radius of Neutron-Deficient

The nuclear root-mean-square charge radius of ^{54}Ni was determined with collinear laser spectroscopy to be R(^{54}Ni)=3.737(3) fm. In conjunction with the known radius of the mirror nucleus ^{54}Fe, the difference of the charge radii was extracted as ΔR_{ch}=0.049(4) fm. Based on the correlation between ΔR_{ch} and the slope of the symmetry energy at nuclear saturation density (L), we deduced 21≤L≤88 MeV. The present result is consistent with the L from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1σ error bands. Our result indicates the neutron-skin thickness of ^{48}Ca as 0.15-0.21 fm.

Motor-driven autonomous system for controlling beamline iris diaphragm apertures.

At the collinear apparatus for laser spectroscopy and applied sciences (COALA) at TU Darmstadt, collinear laser spectroscopy is applied to perform high-precision measurements of atomic transition frequencies and high voltages in an ultra-high vacuum beamline. In such laser spectroscopy beamlines, iris diaphragms are used to reduce ion beam divergence, and to ensure a good laser and ion beam overlap. Since the system that controls the diameter of the apertures presents strong hysteresis, an automated adjustment is desirable to enhance reproducibility in the aperture settings, and to reduce the effort in performing measurements where different diameters are required, especially in alternating collinear and anti-collinear measurements. To achieve this, the Iris Mover system was designed and implemented. The Iris Mover system consists of motor-driven iris apertures which can be easily controlled by users through a computer, and which accounts for hysteresis effects. Here, we explain the design process of the Iris Mover and demonstrate and discuss its functionality.

Application of the Continuous Stern-Gerlach Effect for Laser Spectroscopy of the

We report on the successful demonstration of a novel scheme for detecting optical transitions in highly charged ions. We applied it to determine the frequency of the dipole-forbidden 2p ^{2}P_{1/2}-^{2}P_{3/2} transition in the fine structure of ^{40}Ar^{13+} using a single ion stored in the harmonic potential of a Penning trap. Our measurement scheme does not require detection of fluorescence, instead it makes use of the continuous Stern-Gerlach effect. Our value of 679.216464(4)_{stat}(5)_{syst} THz is in reasonable agreement with the current best literature values and improves its uncertainty by a factor of 24.

Nuclear Charge Radii of

The first laser spectroscopic determination of the change in the nuclear charge radius for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-accuracy measurement of the isotope shift in the 2s^{2}2p ^{2}P_{1/2}→2s^{2}3s ^{2}S_{1/2} ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes ^{10,11}B and the results are used to extract their difference in the mean-square charge radius ⟨r_{c}^{2}⟩^{11}-⟨r_{c}^{2}⟩^{10}=-0.49(12) fm^{2}. The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used to benchmark new ab initio nuclear structure calculations using the no-core shell model and Green's function Monte Carlo approaches. These results are the foundation for a laser spectroscopic determination of the charge radius of the proton-halo candidate ^{8}B.

New Nuclear Magnetic Moment of

A recent measurement of the hyperfine splitting in the ground state of Li-like ^{208}Bi^{80+} has established a "hyperfine puzzle"-the experimental result exhibits a 7σ deviation from the theoretical prediction [J. Ullmann et al., Nat. Commun. 8, 15484 (2017)NCAOBW2041-172310.1038/ncomms15484; J. P. Karr, Nat. Phys. 13, 533 (2017)NPAHAX1745-247310.1038/nphys4159]. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment (µ_{I}) of ^{209}Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of µ_{I}(^{209}Bi) and combine it with nuclear magnetic resonance measurements of Bi(NO_{3})_{3} in nitric acid solutions and of the hexafluoridobismuthate(V) BiF_{6}^{-} ion in acetonitrile. The result clearly reveals that µ_{I}(^{209}Bi) is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.

High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED.

Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209Bi82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron-nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209Bi82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.

Non-destructive single-pass low-noise detection of ions in a beamline.

We have conceived, built, and operated a device for the non-destructive single-pass detection of charged particles in a beamline. The detector is based on the non-resonant pick-up and subsequent low-noise amplification of the image charges induced in a cylindrical electrode surrounding the particles' beam path. The first stage of the amplification electronics is designed to be operated from room temperature down to liquid helium temperature. The device represents a non-destructive charge counter as well as a sensitive timing circuit. We present the concept and design details of the device. We have characterized its performance and show measurements with low-energy highly charged ions (such as Ar(13+)) passing through one of the electrodes of a cylindrical Penning trap. This work demonstrates a novel approach of non-destructive, low noise detection of charged particles which is, depending on the bunch structure, suitable, e.g., for ion traps, low-energy beamlines or accelerator transfer sections.

Precision Test of Many-Body QED in the Be+ 2p Fine Structure Doublet Using Short-Lived Isotopes.

Absolute transition frequencies of the 2s 2S{1/2}→2p2P{1/2,3/2} transitions in Be^{+} were measured for the isotopes ^{7,9-12}Be. The fine structure splitting of the 2p state and its isotope dependence are extracted and compared to results of ab initio calculations using explicitly correlated basis functions, including relativistic and quantum electrodynamics effects at the order of mα(6) and mα(7) ⁢ln α. Accuracy has been improved in both the theory and experiment by 2 orders of magnitude, and good agreement is observed. This represents one of the most accurate tests of quantum electrodynamics for many-electron systems, being insensitive to nuclear uncertainties.

Test of time dilation using stored Li+ ions as clocks at relativistic speed.

We present the concluding result from an Ives-Stilwell-type time dilation experiment using 7Li+ ions confined at a velocity of ß=v/c=0.338 in the storage ring ESR at Darmstadt. A Λ-type three-level system within the hyperfine structure of the 7Li+3S1 →3P2 line is driven by two laser beams aligned parallel and antiparallel relative to the ion beam. The lasers' Doppler shifted frequencies required for resonance are measured with an accuracy of <4×10(-9) using optical-optical double resonance spectroscopy. This allows us to verify the special relativity relation between the time dilation factor γ and the velocity ß, γ√1-ß2=1 to within ±2.3×10(-9) at this velocity. The result, which is singled out by a high boost velocity ß, is also interpreted within Lorentz invariance violating test theories.

Trapped ion oscillation frequencies as sensors for spectroscopy.

The oscillation frequencies of charged particles in a Penning trap can serve as sensors for spectroscopy when additional field components are introduced to the magnetic and electric fields used for confinement. The presence of so-called "magnetic bottles" and specific electric anharmonicities creates calculable energy-dependences of the oscillation frequencies in the radiofrequency domain which may be used to detect the absorption or emission of photons both in the microwave and optical frequency domains. The precise electronic measurement of these oscillation frequencies therefore represents an optical sensor for spectroscopy. We discuss possible applications for precision laser and microwave spectroscopy and their role in the determination of magnetic moments and excited state life-times. Also, the trap-assisted measurement of radiative nuclear de-excitations in the X-ray domain is discussed. This way, the different applications range over more than 12 orders of magnitude in the detectable photon energies, from below µeV in the microwave domain to beyond MeV in the X-ray domain.