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News Archive 2016

29.10.16: Improved understanding of β-decay of 52Ni by high-precision isochronous mass spectrometry

The masses of 52g,52mCo were measured by X. Xu et al. with an unprecedented precision for short-lived nuclei of ~10 keV by isochronous mass spectrometry at the Cooler-Storage Ring at the Heavy Ion Research Facility in Lanzhou (HIRFL-CSR external Link), China. This allowed for the new assignment of the lowest T=2, Jπ=0+ isobaric analog state (IAS), in 52Co. The masses of the T=2 multiplet derived from the new energy value of the IAS in 52Co fit well into the quadratic form of the Isobaric Multiplet Mass Equation (IMME).
The new results have recently been published in Physical Review Letters. They have an impact on the understanding of β-decay properties of 52Ni, questioning the conventional identification of isobaric analog states from the β-delayed proton emissions.

Please read more in the article ... >

20.06.16: Review article on the cryogenic storage ring CSR

Ion storage rings are unique tools for investigating properties and interactions of atomic and molecular ions. The astrophysically relevant dissociative recombination of molecular ions with electrons can be investigated as well as the de-excitation of internal states in atomic ions and in small to very large molecular systems. Complementary to the so far mainly used magnetic storage rings, electrostatic storage rings are also suited for these studies.
Since electrostatic rings do not restrict the mass-to-charge ratio of the particles at a given energy, ions of heavy atoms and molecules, even clusters and large biomolecules, become available for experimental studies in such devices. Thus, electrostatic operation greatly widens the scope of atomic and molecular physics in ion storage rings.

In a recently published article in Review of Scientific Instruments, R. von Hahn et al. report on the development, construction, and first operation of the novel cryogenic electrostatic storage ring CSR, which was officially inaugurated in May 2016 at MPIK in Heidelberg.

In 2005, a novel storage ring was proposed for collisional and laser-interaction studies over long storage times with fast atomic, molecular, and cluster ion beams. Subsequently, for this special purpose, the CSR has been designed and constructed at MPIK in the past years.
Based on the conceptual goals, the layout of the system is discussed in the review article, highlighting its ion-optical, cryogenic and mechanical design. The CSR is designed to house a wide range of experimental equipment.

In March 2014, the storage ring (circumference 35 m) has been put into operation, storing an 40Ar+ ion beam at room temperature. In 2015, the CSR was put into operation at cryogenic temperatures for the first time. It was successfully cooled down to ~6 K and has stored anion and cation beams in the 60 keV range with beam decay time constants up to about an hour. E. g. hydroxide ions (OH-) were cooled down to temperatures as they occur in interstellar space and stored for several minutes inside the CSR.
The observed beam lifetimes are by far long enough for most envisaged experiments in atomic, molecular, and cluster physics. For molecular anions of higher mass, such as the silver dimer Ag2-, an average lifetime as long as 45 min was observed for the stored ions. For future experiments, the ion optical layout of the CSR will allow the beam energy to be increased to 300 keV for singly charged ions.

The results of the detailed performance study discussed in the article as well as the recently performed first CSR experiment with an organic interstellar molecule ion (CH+) underline that this novel electrostatic storage ring will be a powerful instrument for a broad range of physics experiments with fast beams of atomic, molecular, and cluster ions over long observation times in a cryogenic environment and at extremely low residual-gas density (below 140 cm−3, equivalent to a room-temperature pressure below 10−14 mbar).

Please read more in the article ... >

Further information also in the press release of the MPIK external Link.

06.05.16: Collinear laser spectroscopy finds evidence of shape coexistence near doubly magic 78Ni

The protons and neutrons in atomic nuclei occupy quantum levels that are separated by energy gaps leading to the simple nuclear shell model. It successfully describes the nuclear structure near the valley of stability. However, exotic isotopes far from stability, which became accessible experimentally with the development of accelerators and isotope separators, reveal interesting new nuclear structure phenomena. Observations such as the coexistence of different nuclear shapes near closed proton or neutron shells challenge the persistence of the established "magic numbers" of the standard model and lead to theoretical improvements. An example is the concept of "shape coexistence", where normal near-spherical and deformed structures coexist in the atomic nucleus at low energy. The deformed structures, the so called intruder configurations, appear as multi-particle multi-hole (mp-mh) excitations of protons or neutrons across a "closed" shell gap. Shape coexistence at low energy appears all over the nuclear chart, always along closed shells and away from the doubly-magic isotopes.
Experimental evidence for shape coexistence e.g. has already been observed along N = 20, along N = 28 as well as along the sub-shell gap N = 40. Along the next neutron closed shell N = 50, experimental evidence of shape coexistence has not been reported so far.
The magnetic moment μ, and in particular the related g-factor (g = μ/I), is an important probe of the single-particle nature of nuclear states, while the isomer shift provides information on the relative mean square charge distribution of isomeric states. The combination of both observables can be used to identify and characterize shape coexisting structures.

In a recently in Physical Review Letters published article X. F. Yang et al. report on the first measurement of the hyperfine structure (hfs) of the ground and isomeric states of 79Zn by collinear laser spectroscopy. The hfs measurements were carried out at the collinear laser spectroscopy setup COLLAPS at ISOLDE-CERN, Geneva. The hfs in the 4s4p 3P2 → 4s5s 3S1 transition of the ground state and isomeric state in 79Zn was scanned in order to determine the nuclear spins and moments (resp. the related g-factors) of 79gZn and 79mZn as well as the isomer shift resp. the difference of their rms charge radii.
The observed hfs spectrum of the isomeric state of 79Zn establishes a long-lived nature of the isomer with a half-life of more than 200 ms for the first time.
According to the hfs resonance peaks, the spins I = 9/2 and I = 1/2 can firmly be assigned to the ground and isomeric states. The observed ground-state magnetic moment of 79Zn, μ (79Zn) = −1.1866(10) μN, confirms the spin-parity 9/2+ with a ν1g9/2-1 shell-model configuration, in excellent agreement with the prediction from large scale shell-model theories. The magnetic moment μ (79mZn) = −1.0180(12) μN resp. the large negative g-factor for the I = 1/2 isomer 79mZn are consistent with a positive parity and a 2h-1p intruder configuration (however, mp-mh configurations, e.g. 4h-3p, can also not be excluded).
A remarkable result of the measurements is the large isomer shift observed for 79mZn, which proves a large increase of the rms charge radius with respect to its ground state, pointing to a larger deformation. This large deformation can be explained by the intruder (mp-mh) nature of the isomeric state. It provides the first experimental evidence of shape coexistence in 79Zn.
Since shape coexistence normally appears away from the doubly-magic isotopes these new results challenge the magicity of the supposedly "doubly-magic" 78Ni. Thus, further theoretical and experimental investigations are needed in this region of the nuclear chart.

Please read more in the article ... >

PRL Editor's Suggestion
The article has been selected as "Editor's Suggestion". This is an award "based on the potential interest in the results presented and, importantly, on the success of the paper in communicating its message, in particular to readers from other fields." (see also here external Link)

17.03.16: Novel Cryogenic Storage Ring CSR enables the investigation of molecular reactions under interstellar conditions

More than 180 different molecules have been identified in interstellar space to date, among them complex organic molecules and molecular ions. This molecular complexity is surprising, because the conditions in interstellar space (low temperatures combined with extremely low pressure) are rather unfavorable for chemical reactions. Therefore, the chemistry within interstellar clouds, many light-years away from earth, must differ considerably from the well-known terrestrial chemistry. Since a direct investigation of these reactions in space is impossible, they have to be described by theoretical models. A major goal of Laboratory Astrophysics is to experimentally investigate the formation and stability of molecules under interstellar conditions in order to test these models. For this and other purposes, the novel cryogenic electrostatic storage ring CSR (circumference 35 m) has been designed and constructed at MPIK. In May 2015 the researchers at MPIK were able to store and cool hydroxide ions (OH) to interstellar temperatures inside the CSR. This marked the first step toward experiments with astrophysically relevant molecular ions at the CSR.

In a recent article published in Physical Review Letters the CSR collaboration together with the ASTROLAB group external Link, led by Holger Kreckel, report on the first experiment with an organic interstellar molecule in the ultracold storage ring CSR. An important process in interstellar clouds is the interaction of molecular ions with the UV radiation of neighbouring stars. With the present experiment the scientists studied the fragmentation of the CH+ molecular ion by ultraviolet laser light (a process referred to as photodissociation). To this end the stored CH+ ion beam was superimposed with a pulsed ultraviolet laser and the neutral hydrogen atoms that are released through the photodissociation process were recorded with a detector. The dissociation threshold, i.e. the minimal breakup energy of the chemical bond between C and H, differs for different rotational states of the CH+ ion, which allows for the derivation of the populations of the individual rotational states as a function of the storage time.
Theoretical calculations on photodissociation resonances near the threshold show very good agreement with the experimental results. A temperature of around 20 K yielded the best agreement between the measured data and a theoretical model of the rotational decay. These new results demonstrate the potential of the CSR for experiments with organic molecular ions under true interstellar conditions.

Please read more in the article ... >

Further information also in the detailed press releases of the MPIK external Link resp. the idw external Link.

08.02.16: Unexpected evolution of nuclear sizes in Ca isotopes away from stability challenges nuclear theory

In atomic nuclei the nucleons (protons and neutrons) occupy quantum levels that are separated by energy gaps leading to a simple shell structure similar to the well-known electron shells in the atom. The proton shells and the neutron shells are independent of each other. Each shell can be filled with a certain maximal number of protons resp. neutrons. Completely filled shells define the so called "magic" proton and neutron numbers 2, 8, 20, 28, 50, 82 and 126 in the case of the neutrons. Nuclei with magic proton or neutron numbers are known to be stable and the doubly magic isotopes with magic proton and neutron numbers are particularly stable. Examples for doubly magic nuclei are 4He, 16O, 40Ca, 48Ca and 208Pb.
The calcium isotopic chain is a unique nuclear system to study how protons and neutrons interact inside the atomic nucleus. Calcium (Z=20) is the only element with two stable doubly magic isotopes, 40Ca and 48Ca. Furthermore, recent experimental evidence suggests, that the two short-lived calcium isotopes 52Ca and 54Ca are also doubly magic. This would show that shell effects do not smear out far from the valley of stability.
The radius of an imaginary spherical area where the protons are concentrated is called nuclear charge radius. It is still a challenge for nuclear theory to predict the evolution of the charge radii of the known calcium isotopes and so far a microscopic description has been lacking. According to the shell model the charge radii in doubly magic 52Ca and 54Ca are expected to decrease due to the increase of the binding energy per nucleon (see: F. Wienholtz et al., Nature 498, 346–349 (2013) external Link).
Consequently it is important to measure the charge radii of these radioactive calcium isotopes to test the nuclear shell model for exotic nuclei and to understand how shell structure evolves and impacts the limits of stability. Since a change in the nuclear size between two isotopes gives rise to a shift of the atomic hyperfine structure levels, the change in the nuclear mean-square charge radii can be determined by the measurement of this so called isotope shift.
So far the experimental data of the calcium isotopes have suggested an odd-even staggering of their charge radii and a significant reduction of the charge radius for the supposed doubly magic 52Ca would be expected.

In a just in "Nature Physics" published article R. F. Garcia Ruiz et al. report on the detemination of changes in the charge radii for 40-52Ca isotopes and the first measurements of the charge radii of 49,51,52Ca at the ISOLDE external Link on-line isotope separator at CERN, Geneva. The experimental determination of the charge radii of this three calcium isotopes addressed fundamental questions regarding the size of atomic nuclei as well as the understanding of the possible doubly magic character of 52Ca.
Two major experimental challenges are the extremely small isotope shift and the very low production rates and short lifetimes of the exotic nuclei. Therefore it was neccessary to further enhance the experimental sensitivity without sacrificing high resolution. At ISOLDE high-resolution bunched collinear laser spectroscopy was used with further optimization of photon detection sensitivity and reduction of photon background events. This allowed for the high-resolution study of short-lived calcium isotopes with a sensitivity improved by two orders of magnitude and a measurement of 52Ca with one of the highest sensitivities ever reached by using fluorescence detection techniques.
At COLLAPS external Link, a dedicated beam line for collinear laser spectroscopy experiments, the calcium ions were superimposed with a continuous wave laser beam to scan the hyperfine structure (hfs) in the 4s 2S1/2 → 4p 2P3/2 transition of Ca+ by Doppler tuning of the laser frequency. The isotope shifts were extracted from the hfs experimental spectra and allowed the determination of the corresponding root-mean-square (r.m.s.) charge radii.

The experimental results yield a much smaller increase of the r.m.s. charge radius of 49Ca with respect to 48Ca (δ<r2>48,49 = 0.097(4) fm2) than previously suggested (δ<r2>48,49 ≥ 0.5 fm2). The increase continues towards 52Ca (N=32) which shows a very large charge radius with an increase relative to 48Ca of δ<r2>48,52 = 0.530(5) fm2. Under the assumption that the short-lived 52Ca is a new a doubly magic nucleus a much smaller charge radius was expected.

The new experimental charge radii of the exotic neutron-rich calcium isotopes were compared to results of ab initio calculations with optimized chiral effective field theory (EFT) interactions and other theoretical predictions. The ab initio calculations consistently predict an increase in charge radii for 50,52Ca, but fall short of describing the data. In particular, all current model calculations considerably underestimate the large charge radius of 52Ca. Even ab initio calculations considering the impact of core breaking effects including coupling to the neutrons cannot explain its size. One possible reason for these theoretical shortcomings is that deformed intruder states associated with complex nucleon configurations are not taken into account.
A unified description of the atomic nucleus has to include such effects in order to explain the unexpectedly large charge radius of 52Ca and improve our general understanding of the evolution of nuclear sizes away from stability. The currently planned and prepared experimental studies of isotopes even further away from stability, especially the possibly doubly magic nucleus 54Ca with a subshell closure at neutron number 34, will provide the required experimental data on these neutron-rich exotic atomic nuclei.

Please read more in the "Nature Physics" article ... >

Further information also in the press releases of the MPIK external Link, the TU Darmstadt external Link and the idw external Link.

Further links:

25.01.16: Simple nuclear structure in 111–129Cd revealed by high-resolution laser spectroscopy

In most cases nuclear states are complex and thus their theoretical description has to reduce this complexity to a set of basic concepts like shell structure, pairing, "magic" numbers, and deformation. However, in near-closed-shell nuclei, simple structures may occur as a result of the spherical symmetry breaking up. A candidate is cadmium which incorporates an open shell with two protons less than the magic number Z = 50. The cadmium isotopic chain of 111-129Cd appears to provide one such instance of simplicity.

In a recently in Physical Review Letters published article D. T. Yordanov et al. report on the first determination of isomer shifts in 111-129Cd by high-resolution laser spectroscopy. The collinear laser spectroscopy on the neutron-rich cadmium isotopes towards the magic N = 82 shell was performed at the CERN-ISOLDE external Link facility in Geneva. Of interest were the odd neutron-rich cadmium isotopes whose long-lived isomeric states were readily available as a component in the beam. Ground states and isomers were recorded in the same spectrum allowing for a direct measurement of the isomer shift independent of most experimental uncertainties. Since the studied effects are about 3 orders of magnitude smaller than the hyperfine structure and, in most cases, are appreciably smaller than the natural linewidth, the study could only be conducted with high resolution.
The difference of the nuclear charge radii between ground and isomeric state can be derived from the isomer shift. The mean square charge-radii differences between the 11/2- isomers and the 1/2+ and the 3/2+ ground states in 111-129Cd have been found to follow a distinct parabolic dependence as a function of the atomic mass number. No such clear dependence has ever been observed or discussed before. It is, so far, unique to the cadmium chain, although it may be relevant to other nuclear species under the influence of unique-parity orbitals, namely h11/2 or i13/2.

Furthermore, it was examined whether there is a connection between the found parabolic mass dependence of the isomer shifts and the previously found linear mass dependence of the 11/2- quadrupole moments in 111-129Cd (see news of 07.05.13). A comprehensive description assuming nuclear deformation is found to confirm this assumed relation. It accurately reproduces the radii differences in conjunction with the known quadrupole moments. This intuitive interpretation is supported by covariant density functional theory (CDFT).

Please read more in the article ... >

18.01.16: Most stringent test of the nuclear recoil effect in a magnetic field

102 years after the introduction of the first quantised electron shell model for the atomic structure of nature by Niels Bohr, the dynamics of atomically bound electrons is described by one of the most-precisely tested theories. This theory, bound-state quantum electrodynamics (BS-QED), has been recently tested with unrivalled precision by comparing a theoretical prediction and a high-precision measurement of the bound-electron g-factor of hydrogenlike silicon. Here, theory and experiment agree up to eleven significant digits.

The g-factor, which describes the strength of the magnetic moment of the ion, is an excellent observable for testing theory, as a large variety of effects contribute to this value.

In a recently published article in Nature Communications F. Köhler et al. present a direct test of one of the most fascinating g-factor contributions, the nuclear recoil contribution. Whereas for all other contributions the nucleus is treated as a fixed charge distribution in space, the recoil contribution considers the motility of the nucleus in elaborate BS-QED calculations and thus allows to observe the relativistic dynamics of the system. Featuring very similar nuclear charge radii and a 20% mass difference, the tiny (10-8) bound-electron g-factor difference between the two lithiumlike calcium isotopes 40Ca17+ and 48Ca17+ provides a unique test system for the relativistic recoil contribution.

The test has been made possible by the close collaboration of three different physical disciplines: (1) state-of-the-art BS-QED calculations, (2) high-precision measurements of atomic masses and (3) high-precision measurements of Larmor-to-cyclotron frequency ratios.

(1) Theorists from St Petersburg around Vladimir M. Shabaev performed the required BS-QED calculations. They improved and extended the elaborate theoretical framework of lithiumlike ions.

On the experimental side, the g-factors of both calcium isotopes have been determined separately, requiring ultra-precise measurements of their masses as well as their Larmor-to-cyclotron frequency ratios, Γ≡νLc.

(2) At the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, the atomic mass of 48Ca has been determined to 10 significant digits, a prerequisite for the experimental g-factor determination. Here, the cyclotron-frequency ratio of the mass doublet of singly charged 48Ca+ ions and 12C4+ carbon cluster ions has been measured with highest precision by the Penning-trap mass spectrometer SHIPTRAP external Link based on the novel phase-imaging ion-cyclotron resonance technique (PI-ICR).
(3) Both Larmor-to-cyclotron frequency ratios have been measured with highest precision (δΓ/Γ ≈ 5·10-11) at the g-factor experiment for highly charged ions located in Mainz. Here, both calcium ions are subsequently and singly confined in a cryogenic Penning trap apparatus. The setup contains three cylindrical traps: (i) Embedded in a miniature electron beam ion source, the enriched calcium target and the creation trap enable an in situ production of both calcium isotopes. (ii) In the precision trap the cyclotron frequency, νc, of the ion is determined by measuring the frequencies of all three harmonic eigenmotions. These oscillations are measured non-destructively detecting the induced image current via a superconductive resonator. Simultaneously to the phase-sensitive measurement of the modified cyclotron frequency, also the Larmor frequency, νL, is scanned in this trap. (iii) Based on the continuous Stern-Gerlach effect the spin-state of the ion is studied in a third Penning trap. In this so-called analysis trap spin quantum jumps are resolved by coupling the ion's magnetic moment to the axial motion via a large magnetic inhomogeneity. The complete measurement process requires single ion confinement for several months and thus a vacuum of better than 10-16 mbar.

The good agreement between the theoretical predicted relativistic recoil contribution and the high-precision g-factor measurements sets the basis of a new generation of BS-QED tests and paves the way of further fundamental measurements in atomic physics, e.g. the determination of the fine-structure constant α via bound-electron g-factors of heavy, highly charged ions.

Please read more in the "Nature Communications" article ... >

Further information also in the press releases of the MPIK external Link, the idw external Link and the GSI Helmholtzzentrum für Schwerionenforschung external Link.