Isotope Shift Measurements of Li-like Nd-Nd by Means of Dielectronic Recombination

As has been detailed in [1, 2], dielectronic recombination (DR) of few-electron heavy-ions is a promising novel approach to study nuclear properties like charge radii and hyperfine effects. Isotope shift measurements of highly charged ions by means of DR complement nicely optical (laser) spectroscopic techniques in neutral atoms or moderately charged ions since the interpretation of the energy shift is not hampered by many-body effects. A combined analysis offers the opportunity to link together highly accurate data from Laser measurements with the DR isotope shift data and hence to significantly improve the overall accuracy. In contrast to experiments with muonic atoms and—up to now—the technique of high-energy electron scattering, DR isotope shift experiments with unstable exotic species seem feasible. In August 2005, a first pilot experiment with the two stable Li-like neodymium isotopes Nd with A=142 and A=150 was successfully performed at the electron cooler of the ESR. Special care has been taken to ensure identical experimental conditions in the individual runs with the two different isotopes. In particular, this concerned the mutual alignment of electron and ion beam in the electron cooler, cooling and measurement cycles, and the sequences of measurement voltage ramps applied. Figure 1 nicely demonstrates the sensitivity of DR to nuclear size effects. Shown are the energetically lowest resonance groups with 1s2p1/218`j configurations. The spectrum in Figure 1 was obtained within ∼1 hour of data acquisition for each isotope with less than 10 particles stored in the ring. The isotope shift, which is clearly visible, is mostly due to the change in the nuclear size contribution to the 2s − 2p1/2 excitation energy. The captured, loosely bound n = 18 Rydberg electron can be considered as a mere spectator that does not contribute to the energy shift. Corrections like normal as well as specific mass shifts and radiative contributions are small and can easily be accounted for. It should be emphasized that the spectrum only depicts a small fraction of the experimental data. The next higher Rydberg states with n = 19, 20, . . . and resonances from the 2s → 2p3/2 excitation have also been recorded and show corresponding isotope effects. Nd is a semi-magic nucleus and thus possesses a spherical shape. With increasing number of neutrons the nucleus becomes Figure 1: Preliminary experimental data on dielectronic recombination of the two Li-like neodymium Nd isotopes with A=142 (grey line) and A=150 (black line). The initial dielectronic capture populates intermediate doubly excited states, i.e., e + Nd57+(1s2s1/2) → Nd56+(1s2p1/218`j).