Showing papers by "Lutz Schweikhard published in 2013"

Masses of exotic calcium isotopes pin down nuclear forces

Abstract: The properties of exotic nuclei on the verge of existence play a fundamental part in our understanding of nuclear interactions. Exceedingly neutron-rich nuclei become sensitive to new aspects of nuclear forces. Calcium, with its doubly magic isotopes (40)Ca and (48)Ca, is an ideal test for nuclear shell evolution, from the valley of stability to the limits of existence. With a closed proton shell, the calcium isotopes mark the frontier for calculations with three-nucleon forces from chiral effective field theory. Whereas predictions for the masses of (51)Ca and (52)Ca have been validated by direct measurements, it is an open question as to how nuclear masses evolve for heavier calcium isotopes. Here we report the mass determination of the exotic calcium isotopes (53)Ca and (54)Ca, using the multi-reflection time-of-flight mass spectrometer of ISOLTRAP at CERN. The measured masses unambiguously establish a prominent shell closure at neutron number N = 32, in excellent agreement with our theoretical calculations. These results increase our understanding of neutron-rich matter and pin down the subtle components of nuclear forces that are at the forefront of theoretical developments constrained by quantum chromodynamics.

Phase-Imaging Ion-Cyclotron-Resonance Measurements for Short-Lived Nuclides

Abstract: A novel approach based on the projection of the Penning-trap ion motion onto a position-sensitive detector opens the door to very accurate mass measurements on the ppb level even for short-lived nuclides with half-lives well below a second. In addition to the accuracy boost, the new method provides a superior resolving power by which low-lying isomeric states with excitation energy on the 10-keV level can be easily separated from the ground state. A measurement of the mass difference of ^{130}Xe and ^{129}Xe has demonstrated the great potential of the new approach.

Plumbing Neutron Stars to New Depths with the Binding Energy of the Exotic Nuclide Zn 82

Abstract: Modeling the composition of neutron-star crusts depends strongly on binding energies of neutron-rich nuclides near the N = 50 and N = 82 shell closures. Using a recent development of time-of-flight mass spectrometry for on-line purification of radioactive ion beams to access more exotic species, we have determined for the first time the mass of (82)Zn with the ISOLTRAP setup at the ISOLDE-CERN facility. With a robust neutron-star model based on nuclear energy-density-functional theory, we solve the general relativistic Tolman-Oppenheimer-Volkoff equations and calculate the neutron-star crust composition based on the new experimental mass. The composition profile is not only altered but now constrained by experimental data deeper into the crust than before.

New developments of the in-source spectroscopy method at RILIS/ISOLDE

Abstract: At the CERN ISOLDE facility, long isotope chains of many elements are produced by proton-induced reactions in target materials such as uranium carbide. The Resonance Ionization Laser Ion Source (RILIS) is an efficient and selective means of ionizing the reaction products to produce an ion beam of a chosen isotope. Coupling the RILIS with modern ion detection techniques enables highly sensitive studies of nuclear properties (spins, electromagnetic moments and charge radii) along an isotope chain, provided that the isotope shifts and hyperfine structure splitting of the atomic transitions can be resolved. At ISOLDE the campaign to measure the systematics of isotopes in the lead region (Pb, Bi, Tl and Po) has been extended to include the gold and astatine isotope chains. Several developments were specifically required for the feasibility of the most recent measurements: new ionization schemes (Po, At); a remote controlled narrow line-width mode of operation for the RILIS Ti:sapphire laser (At, Au, Po); isobar free ionization using the Laser Ion Source Trap, LIST (Po); isobar selective particle identification using the multi-reflection time-of-flight mass separator (MR-ToF MS) of ISOLTRAP (Au, At). These are summarized as part of an overview of the current status of the in-source resonance ionization spectroscopy setup at ISOLDE.

Recent exploits of the ISOLTRAP mass spectrometer

Abstract: The Penning-trap mass spectrometer ISOLTRAP, located at the isotope-separator facility ISOLDE (CERN), is presented in its current form taking into account technical developments since 2007. Three areas of developments are presented. The reference ion sources have been modified to guarantee a sufficient supply of reference ions for mass measurements and systematic studies. Different excitation schemes have been investigated for manipulation of the ion motion in the Penning trap, to enhance either the purification or measurement process. A multi-reflection time-of-flight mass separator has been implemented and can now be routinely used for purification and as a versatile tool for beam analysis.

Data analysis of Q-value measurements for double-electron capture with SHIPTRAP

Abstract: A measurement campaign has been carried out for the search for resonantly enhanced neutrinoless double-electron-capture transitions by the determination of the Q ee -values with the SHIPTRAP Penning-trap mass spectrometer. The Q ee -values have been determined by measuring the cyclotron-frequency ratios of the mother and daughter nuclides of the transitions. This article describes the experimental approach and the data analysis by the example of neutrinoless double-electron capture in 152Gd. Various effects as, e.g., temporal fluctuations and spatial inhomogeneity of the magnetic field, or the variation of the ion number in the trap were found not to affect the frequency ratio on the 1 ppb-level, which is the present statistical uncertainty.

Towards systematic investigations of space-charge phenomena in multi-reflection ion traps

Abstract: A multi-reflection time-of-flight mass spectrometer has been set up for systematic studies of Coulomb effects of stored ion bunches. We report preliminary experimental results and simulations of peak coalescence of time-of-flight signals as a function of the number of simultaneously trapped ions.

Appearance size of poly-anionic aluminum clusters, Al n z- , z = 2–5*

Abstract: The production of poly-anionic metal clusters by simultaneous storage of electrons and cluster anions in a Penning trap has been extended to the fifth charge state. The minimum cluster size, required to attach a fifth excess electron, has been experimentally determined for aluminum clusters. A refined data evaluation method is proposed, redefining the appearance size with respect to the delayed electron emission. It has been applied to the penta-anions as well as to previous data of poly-anionic aluminum clusters. In addition, new measurements of aluminum di-anions have revealed a lower minimum appearance size than reported earlier. Comparison of the experimental results with predictions by the conducting-sphere model for the di-, tri-, tetra- and penta-anions show deviations that are probably due to thermal excitation of the cluster anions. The cluster-size dependence of the poly-anion abundance spectra is qualitatively reproduced by thermionic emission.

Production of multiply-charged metal-cluster anions in Penning and radio-frequency traps

Abstract: The number of electrons in an atomic cluster can have a severe influence on its properties, and thus the charge state is a crucial parameter. In the present study, poly-anionic clusters are produced by electron-attachment to cluster mono-anions stored in ion storage devices. The poly-anion production is investigated for two different trap types. In a Penning trap (ion-cyclotronresonance, or short ICR trap) charged particles with any mass-to-charge ratio up to a critical value can be stored at the same time. Thus, cluster anions are stored together with electrons, simultaneously, allowing for electron attachment from an electron bath. However, the highest poly-anionic charge state that can be produced is limited by two conditions. On the one hand, it is restricted by the upper mass limit of the trap, because a minimum cluster size is required to gain a certain (negative) charge state. On the other hand, it is limited by the Coulomb potential of the cluster anions, because the maximum energy of the attaching electrons in the bath is given by the depth of the trapping potential. In contrast to the Penning trap, the radio-frequency (RF) traps confine particles with only a narrow range of the mass-over-charge ratio values. In particular, there is a lower limit, preventing a simultaneous storage of electrons and clusters. At the same time, the RF trap's mass range can be shifted to large clusters, which are required to reach higher negative charge states. Application of rectangularly shaped RF-voltages, in contrast to sinusoidal ones, facilitates ion storage with respect to variation of the mass range, i.e. cluster size range, to be trapped. While poly-anion production in Penning traps is a well-established technique that has already been applied to several types of clusters, its realization in RF traps is currently being developed. Both schemes are described, including previous and recent experimental results.

First experiments with the Greifswald electron-beam ion trap

Abstract: The former Berlin electron-beam ion trap (EBIT) was moved to Greifswald. In addition to x-ray studies the setup will be used for the investigation of interaction processes between highly charged ions and atomic clusters such as charge exchange and fragmentation. The EBIT setup has now been reassembled and highly charged ions have been produced from Xe–Ar gas mixtures to study the 'sawtooth effect'. In addition, the layout of the extraction beamline, the interaction region and product analysis for interaction studies with highly charged ions are presented.

Investigations of sawtooth oscillations with the Greifswald EBIT

Abstract: The former Berlin electron-beam ion-trap was moved to Greifswald. One of the first aims after the reinstallation was the continuation of experiments using mixed ensembles of low- and high-Z ions for further studies of the previously reported sawtooth-like oscillations of the trap plasma. First results of these studies for xenon/argon mixtures are presented.

Kernstruktur von Exoten

Abstract: Die Massen von Radionukliden sind wichtig, um Kernmodelle zu testen oder astrophysikalische Fragestellungen, wie den Aufbau von Neutronensternen, zu beantworten. Aufgrund ihrer oft geringen Halbwertszeit lassen sie sich jedoch nur schwer messen. Kurzlich gelang es der ISOLTRAP-Gruppe am Forschungszentrum CERN, die Massen von 54Ca und 82Zn mit hoher Genauigkeit zu bestimmen. Damit erhalt man Aufschluss uber das Wirken von Dreikorperkraften innerhalb des Atomkerns sowie uber die Elemententstehung in der Kruste von Neutronensternen.

Schaleneffekte in den schwersten Elementen

Abstract: Am GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt ist erstmals die direkte Messung der Starke von Schaleneffekten in sehr schweren Atomkernen mit uber 100 Protonen gelungen. Durch hochprazise Massenmessungen an den Transuranen Nobelium und Lawrencium konnten diese Schaleneffekte fur 152 Neutronen bestimmt werden. Die Ergebnisse liefern Informationen uber die Kernstruktur superschwerer Elemente und dienen dazu, die Vorhersagen uber Lage und Ausdehnung der “Insel der Stabilitat” zu verbessern.