G. Ewald

Nuclear Charge Radii of 9,11Li: The Influence of Halo Neutrons

The nuclear charge radius of 11Li has been determined for the first time by high-precision laser spectroscopy. On-line measurements at TRIUMF-ISAC yielded a 7Li-11Li isotope shift (IS) of 25 101.23(13) MHz for the Doppler-free [FORMULA: SEE TEXT]transition. IS accuracy for all other bound Li isotopes was also improved. Differences from calculated mass-based IS yield values for change in charge radius along the isotope chain. The charge radius decreases monotonically from 6Li to 9Li, and then increases from 2.217(35) to 2.467(37) fm for 11Li. This is compared to various models, and it is found that a combination of halo neutron correlation and intrinsic core excitation best reproduces the experimental results.

Isotope Shift Measurements of Stable and Short-Lived Lithium Isotopes for Nuclear Charge Radii Determination

Changes in the mean square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8-ms-lifetime isotope with production rates on the order of only 10 000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope {sup 11}Li at the on-line isotope separators at GSI, Darmstadt, Germany, and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.

A setup for high-resolution isotope shift measurements on unstable lithium isotopes

Abstract We present a laser spectroscopic approach for measuring the charge radius of the halo nucleus 11 Li and report on recent progress in the development of the experimental apparatus.

Absolute frequency measurements on the 2S ! 3S transition of lithium-6,7

The frequencies of the 2S–3S two-photon transition for the stable lithium isotopes were measured by cavity-enhanced Doppler-free laser excitation that was controlled by a femtosecond frequency comb. The resulting values of 815 618 181.57(18) and 815 606 727.59(18) MHz, respectively, for 7Li and 6Li are in agreement with previous measurements but are more accurate by an order of magnitude. There is still a discrepancy of about 11.6 and 10.6 MHz from the latest theoretical values. This is comparable to the uncertainty in the theoretical calculations, while uncertainty in our experimental values is more than a hundred-fold smaller. More accurate theoretical calculation of the transition frequencies would allow extraction of the absolute charge radii for these stable isotopes, which in turn could improve nuclear charge radii values for the unstable lithium isotopes.

Towards a precision test of time dilation at high velocity

We report on first measurements towards an Ives–Stilwell test of time dilation at velocities around 0.3c. In Ives–Stilwell type experiments, fast atomic ions containing a well-known transition are used as moving clocks, and time dilation as well as the velocity can be derived from the simultaneous laser-spectroscopic measurements of the Doppler shifts with and against the direction of motion. To accurately measure these Doppler shifts, the Doppler broadening caused by the ions velocity distribution needs to be overcome. We performed first feasibility studies for laser spectroscopy on 7Li+ ions in the 2s3S21 metastable ground state at the Gesellschaft fur Schwerinonenforschung (GSI) in Darmstadt. The ions were stored in the Experimental Storage Ring (ESR) at a velocity of 0.338c, and optical–optical double-resonance spectroscopy on a closed Λ-type three level system was performed with two lasers propagating antiparallel to the ions motion. We found that Doppler shift measurements on a narrow subclass of th...

Nuclear and electron polarization contributions to the HFS of hydrogen- and lithium-like ions

Hydrogenlike high-Z atoms offer fascinating possibilities for testing the interplay of nuclear and atomic structure. They have recently become available for experiments at GSI in Darmstadt [1] and at the LLNL in Livermore [2, 3] and have been used to test the theory of quantum electrodynamics (QED) in extremely strong electric and magnetic fields. Measuring the ground-state hyperfine structure (HFS) of these hydrogenlike systems is a sensitive method to explore QED and nuclear contributions to the electron energy. Experiments at GSI included laser spectroscopic investigations of Bi and Pb at the heavy-ion storage ring [1]. These measurements, with a relative accuracy < 10−3, allowed the first test of QED in the strong electromagnetic field of highly-charged heavy ions. At Livermore, hydrogenlike Ho [2], Re, and Re [2], Tl, and Tl [3] have been produced and stored in a high-energy electron-beam ion trap (SuperEBIT) by an energy variable electron beam, axially compressed by a strong magnetic field. In this contribution we present results of calculations performed for the hyperfine splitting by using the Dynamic Correlation Model (DCM) [4]. The DCM describes the structure of open-shell nuclei with an odd number of valence particles in terms of clusters: the valence and the core cluster. In contrast to Hartree-Fock approaches the DCM includes a coupling mechanism acting between the core and the valence particles, which modifies the Hartree-Fock (HF) description and polarizes the core via particle-hole excitations (2h̄ω) of protons and neutrons. Thus, the valence particle becomes “dressed ” in the sense that it coexists with complex excitations of the core. From a HartreeFock point-of-view, these dressed particles correspond at most to a particular solution of an extended, nonlinear HF Hamiltonian. DCM corrections applied to hyperfine splitting calculations [5] show good agreement with experimental data [1, 2, 3] if the QED corrections [6,7] are neglected, however, by adding these corrections a systematic deviation between theory and experiment has been observed. In order to clarify this open point an analysis of the nonlinear terms contributing to the HFS was provided in [5] by performing a term-to-term comparison with the results of other theoretical models [6, 8]. Since the calculated Bohr-Weisskopf term ( ) is larger than that obtained in perturbation theories [8, 6], the reason for this systematic disagreement could be associated with the modification of the single-hole magnetization distribution as shown in Fig. 1 for the Pb and Tl isotopes. By comparing the HFS of the lithiumlike ions with those calculated for hydrogenlike ions, we obtain further information about the hyperfine interaction of the nucleus with the electronic cloud of high-Z ions. The simplest systems which have a closed K−shell are lithiumlike ions. For the ground state hyperfine transition for lithiumlike bismuth a similar situation was found as for hydrogen-like bismuth. In the zeroth-order, the hyperfine interaction of the nuclear moment with the motion of the electrons is represented by a dipole field with a 1/r behaviour. This behaviour leads to the typically strong dependence of the hyperfine splitting on nuclear parameters.