News Archive 2020

In Penning traps electromagnetic forces are used to confine charged particles under well-controlled conditions for virtually unlimited time. Sensitive detection methods have been developed to allow observation of single stored ions. Various cooling methods can be employed to reduce the energy of the trapped particle to nearly at rest.

In a recent review article published in Quantum Science and Technology, K. Blaum, S. Eliseev and S. Sturm summarize how highly charged ions (HCIs) offer unique possibilities for precision measurements in Penning traps. Precision atomic and nuclear masses as well as magnetic moments of bound electrons allow among others to determine fundamental constants like the mass of the electron or to perform stringent tests of fundamental interactions like bound-state quantum electrodynamics.
The authors discuss recent results and future perspectives in high-precision Penning-trap spectroscopy with HCIs.

Please read more in the review article ... >

The electron, the proton and the neutron are fundamental building blocks of matter. Their masses and the masses of their simplest combinations forming the lightest atomic nuclei provide quantitative links between a multitude of observables in atoms and molecules.
E.g., the mass of the proton m_p is essential in hydrogen and muonic hydrogen spectroscopy, as it is required for the determination of the Rydberg constant or the proton radius with high precision. Furthermore, the proton-electron mass ratio m_p/m_e is a central parameter in various atomic physics experiments. The mass of the deuteron (atomic nucleus consisting of one proton and one neutron) in combination with its measured nuclear binding energy enables the mass of the neutron to be determined with high precision. The highly precise mass difference of triton (atomic nucleus consisting of one proton and two neutrons) and helion (atomic nucleus of ³He, consisting of two protons and one neutron) is of high interest for the KATRIN (KArlsruhe TRItium Neutrino) experiment.
At present, 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, the most precisely measured light ion masses from spectrometers around the world show inconsistencies, which can be expressed in terms of the term Δ=m_p+m_d-m_He. This is the so-called "the light ion mass puzzle". This value can either be derived from a mass ratio measurement of ³He⁺ and HD⁺, or from the mass measurements of proton, deuteron and helion in atomic mass units. Both values differ by five standard deviations. This suggests that the uncertainty of these important mass values may have been underestimated.

In a recent article published in "Nature", S. Rau et al. present the results from absolute mass measurements of the deuteron and the HD⁺ molecular ion against ¹²C as a mass reference. The measurements were performed using LIONTRAP (Light ION TRAP), a cryogenic Penning-trap mass spectrometer aiming for most precise mass measurements on various light ions, situated at the Johannes Gutenberg University in Mainz, Germany.
In a Penning trap, the confinement of a single charged particle is achieved by the use of a strong, homogeneous magnetic field that keeps the ion radially confined and an electric quadrupole potential that stores it axially. This yields a motion of the stored ion which can be divided into three harmonic eigenmotions: the modified cyclotron motion with the modified cyclotron frequency ν₊, the very slow magnetron movement with the magnetron frequency ν_- and the oscillation in axial direction with the axial frequency ν_z. The actual cyclotron frequency ν_c=(1/2π)·(q/m)·B of a stored ion with charge q and mass m can be calculated from the invariance theorem: ν_c=(ν₊²+ν_z²+ν_-²)^1/2
In Penning-trap mass ratio measurements, the cyclotron frequency ν_c of the ion under investigation is compared to the cyclotron frequency ν_c^ref of a reference ion in the same magnetic field B: m=(q/q_ref)·(ν_c^ref/ν_c)·m_ref
Thus, the mass determinations are based on the high-precision measurement of the cyclotron frequency ratio ν_c^ref/ν_c. Since the atomic mass unit u is one twelfth of the mass of a ¹²C atom, ¹²C was used as mass reference. To measure the mass of deuteron m_d with LIONTRAP, ¹²C⁶⁺ was used as reference ion. For the mass measurement of the HD⁺ molecular ion, ¹²C⁴⁺ was used as reference ion. Ideally, the frequency ratio ν_c^ref/ν_c has to be measured simultaneously at the same position inside the trap in order to nullify the magnetic field fluctuation. At LIONTRAP this ideal measuring principle is approached by using a precision trap (PT) and two separate storage traps (ST-I, ST-II). By shuttling the ion under investigation (deuteron or HD⁺) and reference ion (¹²C⁶⁺ or ¹²C⁴⁺) between the storage traps and the PT, the time between successive cyclotron frequency measurements is minimized. Furthermore, the identical electrical field configurations for both ions guarantee the identical position of the ions in the PT and therewith the same magnetic field for the ν_c^ref/ν_c measurement.
At LIONTRAP, the axial frequency ν_z can be determined by detecting the image currents induced by the axial movement of the ion of interest on the trap electrodes (so-called "dip" spectrum). The modified cyclotron frequency ν₊ and the magnetron frequency ν_- can be measured by sideband coupling. To this end, the axial and the radial modes are coupled via radiofrequency excitations using radio frequency drives on the motional sidebands at ν₊-ν_z and ν_z+ν_- (so-called "double-dip" technique). Since ν₊>>ν_z>>ν_-, the modified cyclotron frequency ν₊ has the highest significance and thus was measured with highest precision using the fast phase-sensitive measurement method PnA (Pulse and Amplify).

The LIONTRAP mass value for the deuteron m_d=2.013553212535(17) u supersedes the precision of the current CODATA (Committee on Data for Science and Technology) 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. The directly measured mass of the HD⁺ molecular ion, m(HD⁺)=3.021378241561(61) u, allowed for a rigorous consistency check of the measurements of the deuteron mass in the present experiment and the proton mass (see our news of 18.07.17). The mass value derived from the masses of proton and deuteron m(HD⁺)_p+d=3.021378241576(37) u agrees with the directly measured one on a one sigma level. This striking agreement among the LIONTRAP measurements with different masses and systematics substantiates the applied measurement methods.
Furthermore, the deuteron-to-proton mass ratio m_d/m_p(LIONTRAP)=1.999007501228(59) agrees with a direct measurement using H₂⁺ molecular ions recently reported by the Florida State University [Phys. Rev. Lett. 124, 013001 (2020) ] on a one sigma level.
The agreement between the measurements at LIONTRAP and the Florida State University opens up the possibility of a least squares adjustment, reducing the uncertainty of the proton mass m_p by a factor of three compared to the current CODATA value.

Please read more in the "Nature" article  and the associated article in News & Views .

Further information also in the press releases of the MPIK  (idw ) and the Max Planck Society .

Further press releases:

• GSI Helmholtzzentrum für Schwerionenforschung GmbH 
• Johannes Gutenberg-Universität Mainz 
• Spektrum.de - Nachrichten aus Wissenschaft und Forschung 
• scinexx | Das Wissensmagazin 
• pro-physik.de 
• myScience 
• chemeurope.com 
• analytica-world.com 
• WISSEN.NEWZS.de 
• wissenschaftler.de 
• chemiker.de 
• analytiker.de 
• analytik.de 
• Innovations Report 

An international research project with the participation of scientists of the TU Darmstadt, the MPIK Heidelberg, the Friedrich–Alexander University Erlangen–Nürnberg and the Johannes Gutenberg University Mainz performed high-precision measurements in the long isotopic chain of magic tin and determined deviations from the spherical nuclear shape. The laser spectroscopic experiments on the partly very short-lived tin isotopes were conducted at the radioactive ion beam facility ISOLDE  of CERN  in Geneva.

The so-called quadrupole moment Q is a quantitative measure for the nuclear deformation. The recent investigations on tin isotopes revealed that Q does not show a linear trend along the isotopic chain, but a nearly perfect parabolic behavior.
The researchers compared this experimental result with the predictions of the Fayans density functional, which recently could reproduce the trend of charge radii of cadmium isotopes (see our news of 04.09.18). This nuclear model roughly describes the general trend of nuclear deformation of the tin isotopes, but it substantially differs in the details. Thus, the new data can contribute to further improve our understanding of the nuclear forces.

Please read more in the article ... >

Further information (in German) also in the press release of the TU Darmstadt  (idw ).

In June 2020, the article was selected for the Home page of Communications Physics .

Every time measurement using a clock is based on counting periodic processes, e.g. the pendulum oscillation of a pendulum clock. If the number of swings per second is known (or defined), the counting of the number of oscillations allows to determine the elapsed time in seconds. Today, the definition of time is based on counting the oscillations of a transition in an atom with frequencies in the microwave (gigahertz) range. The current international time standard using cesium atomic clocks reaches a fractional precision of 10⁻¹⁶. Thus, advanced atomic clocks gain or lose only a second in over 100 million years.

Optical atomic clocks are the next generation of atomic clocks and have clock frequencies in the optical spectral (terahertz) range. Optical clocks achieve fractional precisions of 10⁻¹⁸ and below using ensembles of atoms in optical lattices or individual ions in radiofrequency traps.
No electronics is fast enough to measure the terahertz frequencies of an optical clock. Nevertheless, the use of frequency combs allows the construction of optical clocks. Frequency combs work like a "frequency gear". They transform the optical terahertz frequencies into electronically measurable gigahertz frequencies. Optical atomic clocks are used as frequency standards and in searches for possible variations of fundamental constants, dark matter detection, and physics beyond the Standard Model.
Promising candidates for a new generation of clocks are highly charged ions (HCIs, see e.g. recent quantum logic spectroscopy with ⁴⁰Ar¹³⁺ [Nature 578, 60-65 (2020) ]) and nuclear transitions (e.g. the ²²⁹Th nuclear clock transition [Nature 573, 243-246 (2019) ]). They are largely insensitive to external perturbations and reach wavelengths beyond the optical range, now becoming accessible to frequency combs.
However, the development of novel HCI clocks depends on the identification of suitable clock transitions in HCIs. The transition detection is hindered by difficulties when using conventional spectroscopy methods and by insufficiently accurate atomic structure calculations.

In a recent article published in "Nature", R. X. Schüssler et al. report on the detection of an extremely long-lived metastable state in highly charged ions (¹⁸⁷Re²⁹⁺) using a unique approach to determine the electronic excitation energy. It was determined by measuring the mass difference of the ground and the excited state of the ¹⁸⁷Re²⁹⁺ ion. The experiment was performed with the novel five-trap Penning trap-system PENTATRAP at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg, Germany. It was the first direct, non-destructive determination of an excitation energy in HCIs using high-precision Penning-trap mass spectrometry.
The mass determination with PENTATRAP is performed by measuring the free cyclotron frequency ν_c=1/(2π)·qB/m of an HCI inside a Penning trap. The cyclotron frequency ν_c can be determined from the three trap frequencies ν_- (magnetron frequency), ν_z (axial frequency) and ν₊ (modified cyclotron frequency) of the HCI motion using the invariance theorem: ν_c² = ν_-² + ν_z² + ν₊².
The unique multi-trap configuration of PENTATRAP consists of five identical cylindrical traps for simultaneous mass measurements. This significantly reduces the uncertainty in the cyclotron frequency-ratio (i.e. mass-ratio) determination. The cyclotron frequency ratio R of ¹⁸⁷Re²⁹⁺ in the ground state to the metastable state could be determined with an unprecedented precision of δR = 1·10⁻¹¹. The calculated energy of the metastable state of ¹⁸⁷Re²⁹⁺ with respect to the ground state is ΔE_Re = 202.2(17) eV.

The experimentally determined excitation energy of the long-lived metastable state was compared to advanced theoretical calculations. These were obtained using Multi-configuration Dirac-Hartree Fock approaches in two different implementations and by means of a configuration-interaction (Quanty) calculation.
The theoretical values and experimental results agree very well. The calculations also helped to identify the observed metastable state [Kr]4d⁹ 4f ³H₅ of the ¹⁸⁷Re²⁹⁺ ion.

With a lifetime of about 130 days, the potential soft x-ray frequency reference at ν = 4.86·10¹⁶ Hz has a linewidth of only Δν ≈ 5·10⁻⁸ Hz, and one of the highest electronic quality factors (Q = ν/Δν ≈ 10²⁴) ever seen in an experiment. This low uncertainty enables searching for more HCI soft x-ray clock transitions using PENTATRAP.

The PENTATRAP project receives funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme under grant agreement No 832848 - FunI .

Please read more in the "Nature" article  and the associated article in News & Views .

Further information also in the press releases of the MPIK  (idw ) and of BASE .

Further press releases:

• FAZ.NET 
• wissenschaft.de 
• EurekAlert! Science News 
• analytica-world.com 
• ScienceDaily 
• WISSEN.NEWZS.de 
• Physik für Alle - Physik-News 

Many areas of fundamental physics require the highly precise knowledge of mass differences or mass ratios of a variety of nuclides, e.g. neutrino physics, the test of special relativity and bound-state quantum electrodynamics (BS-QED), ion clocks and the search for Dark Matter via high-resolution isotope shift measurements. In order to satisfy these requirements, the novel Penning-trap experiment PENTATRAP has been set up at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg. PENTATRAP is based on high-precision Penning-trap mass spectrometry (PTMS). A unique feature of the PENTATRAP mass spectrometer is its multi-trap configuration which consists of five identical cylindrical traps. Such a multi-trap configuration significantly reduces the uncertainty in the mass-ratio determination. PENTATRAP offers a realistic opportunity to pursue a mass-ratio measurement on heavy (A>100) highly charged ions with a fractional uncertainty δm/m below 10^-11.
The mass determination of an ion in a Penning trap is done by measuring its free cyclotron frequency ν_c = 1/(2π) · qB/m, where q/m is the charge-to-mass ratio of the ion and B the magnetic field. The cyclotron frequency ν_c can be determined from the three independent harmonic oscillations ν_- (magnetron frequency), ν_z (axial frequency) and ν₊ (modified cyclotron frequency) of the ion motion in the Penning trap using the invariance theorem: ν_c² = ν_-² + ν_z² + ν₊².

In a recent article published in Physical Review Letters A. Rischka et al. present the first mass measurements carried out with PENTATRAP on stable isotopes of xenon (Z=54) with relative uncertainties close to 1·10^-11. The mass differences of the five pairs of stable xenon isotopes, ¹³⁴Xe-¹³²Xe, ¹³²Xe-¹³¹Xe, ¹³¹Xe-¹²⁹Xe, ¹²⁹Xe-¹²⁸Xe and ¹²⁸Xe-¹²⁶Xe, were determined by measuring the free-cyclotron frequency ratios of the corresponding xenon ions in a charge state of 17+. The high precision was achieved by the use of highly charged ions (q=+17), the long storage time of a single Xe ion and the application of the fast phase-sensitive frequency-measurement technique PnP (Pulse-and-Phase). Furthermore, the cyclotron and the axial frequencies were measured simultaneously for first time in this experiment, which substantially reduced the uncertainty in the determination of the free cyclotron frequency.
The demonstrated ability of PENTATRAP to perform high-precision measurements on an isotope chain of the same element will be applied on isotope chains such as Ca, Sr and Yb, which are of very high importance to laser-spectroscopic experiments for Dark Matter searches.

Moreover, the scientists determined the binding energy of the 37th electron in xenon by measuring the ratio of the free cyclotron frequencies of ¹³¹Xe¹⁸⁺ and ¹³¹Xe¹⁷⁺ ions: ν_c(¹³¹Xe¹⁸⁺)/ν_c(¹³¹Xe¹⁷⁺).
The experimental result was compared with two theory values obtained by using two independent different implementations of the multiconfiguration Dirac-Hartree-Fock method. The measured value of 432.4(1.3)(3.4) eV agrees within one-sigma uncertainty with the theoretical values 432.4(3.0) eV and 435.1(1.0) eV.
The comparison is the result of the great collaboration between the theory division  of Christoph H. Keitel and our division.
This "proof-of-principle" measurement with PENTATRAP opens the door to future measurements of electron binding energies in very highly charged heavy ions (e.g. in hydrogen-like xenon ions) with an uncertainty of an eV required to perform stringent tests of bound-state QED in strong electromagnetic fields.

Please read more in the article ... >

The article has been selected as "Editors' Suggestion".

In atomic nuclei, the protons and neutrons occupy quantum levels that are separated by energy gaps leading to the simple nuclear shell model which is comparable to Bohr's model of the atom. According to the shell model, each shell can be filled with a certain maximal number of protons or neutrons. These so-called "magic" numbers (Z, N=8,20,28,50,82 and N=126) of protons and neutrons are associated with large energy gaps in the effective single-particle spectrum of the nuclear mean field, revealing shell closures.
The magic numbers are intimately connected to the nuclear interaction and thus represent essential benchmarks for nuclear models. In order to test new, improved theoretical models, in recent years, numerous high-precision experiments have been performed for nuclear-structure studies of isotopes and isotopic chains in the vicinity of shell closures.
New, but weaker shell closures have been found by experiments with light exotic nuclei. E.g. a shell closure at N=32 by precision mass measurements of the exotic calcium isotopes ⁵³Ca and ⁵⁴Ca (see our news of 19.06.13) and a shell closure at N=34. Significantly less is known for heavier nuclei, in particular for the magic N=82. Recently, the doubly-magic nature of ¹³²Sn (P=50, N=82) was reconfirmed by high-precision laser spectroscopic measurements (see our news of 16.05.19).
Theoretical predictions for the N=82 shell gap rely on precise experimental data for nuclides with Z<50 and N≈82. In recent years, the neutron-rich cadmium isotopes (Z=48) have been investigated by decay-spectroscopy, high-resolution laser spectroscopy (see our news of 07.05.13) and precision mass measurements (see our news of 04.12.15). However, the energies of the low-lying isomers in ¹²⁹Cd and the N=82 two-neutron shell gap remain unknown.

In a recent article published in Physical Review Letters V. Manea, J. Karthein et al. present the first direct determination of the N=82 shell gap for Z<50 with mass measurements of exotic neutron-rich cadmium isotopes and isomers between ¹²⁴Cd and ¹³²Cd. The experiments were performed with the ISOLTRAP  spectrometer at ISOLDE /CERN , Geneva, exploiting all mass-measurement techniques of ISOLTRAP. Depending on the half-life and production yield of the ion of interest, the mass determination at ISOLTRAP can be performed either by the time-of-flight ion cyclotron resonance technique (ToF-ICR) using the precision Penning trap or by performing time-of-flight mass spectrometry with the multireflection time-of-flight mass separator (MR-ToF MS, see, for example, our news of 19.06.13).

The masses of ^131,132Cd were determined with the MR-ToF MS. The masses of the other studied cadmium isotopes were determined with the precision Penning trap by measuring their cyclotron frequency (as singly charged ions) in the trap.
The Penning-trap measurements of ^{124,126,128,131}Cd were performed with the ToF-ICR method, including Ramsey-type excitations. For ^127,129Cd the ion beam was a mixture of ground and isomeric state (J = 3/2⁺ and J = 11/2^-). In 2015, these two states could not be separated by a long-excitation ToF-ICR measurement due to the short half-lives. In the present work, the recently developed phase-imaging ion-cyclotron-resonance (PI-ICR) method was used instead (see our news of 19.02.13 and 11.02.14) to separate the 11/2^- and 3/2⁺ states of ¹²⁹Cd. The ordering of the low-lying isomers in ¹²⁹Cd and their energies were determined, establishing the inversion of the 11/2^- and 3/2⁺ states in ¹²⁹Cd. It shows that the h_11/2 neutron orbital is key for the evolution of the N=82 shell gap towards Z=40.

The new mass of ¹³²Cd allowed addressing a broader range of nuclear models via determination of the N=82 two-neutron shell gap Δ_2n(Z, N) = S_2n(Z, N) - S_2n (Z, N + 2) (S_2n: two-neutron separation energy). This quantity involves only even nuclei and ¹³²Cd (Z=48, N=84) yields the first Δ_2n(Z, N) value below the doubly magic ¹³²Sn (Z=50, N=82). The new data reveal a peak of Δ_2n(Z, N) at the proton magic number Z=50 and thus confirm the phenomenon of "mutually enhanced magicity" at ¹³²Sn (i.e. increased stability associated with neutron magic number when the proton number is magic, and vice versa).

The measured data were interpreted in comparison to the large-scale shell model and to new calculations made with a beyond-mean-field (BMF) approach, as well as the ab initio valence-space in-medium similarity renormalization group (VS-IMSRG). The BMF model reproduces the effect, but underestimates the size of the drop of Δ_2n(Z, N) below Z<50. The VS-IMSRG approach shows an off-set to experiment, but describes the peak at the proton magic number Z=50 qualitatively.

Please read more in the article ... >