Ultrasensitive radioactive detection of collinear-laser optical pumping: Measurement of the nuclear charge radius ofCa2

Abstract

We report the first application of an ultrasensitive detection scheme in on-line collinear-laser spectroscopy. It is based on radioactive detection of optical depopulation pumping, using state-selective charge exchange as an intermediate step. This extends the calcium isotope shift measurements beyond the fm shell closure. The extracted ^'Ca mean-square charge radius constitutes the first experimentally determined one of a short-lived neutron-rich isotope beyond the A^=28 shell closure. The steep increase in radius after the doubly magic "^^^Ca nucleus indicates a strong coupling between the pm neutrons and the core protons.PACS numbers: 21.10.Ft, 27.40.+Z, 32.80. Bx During the past decade, collinear-laser spectroscopy at on-line mass separators has provided a wealth of new information about nuclear spins, moments, and radii by the development and the application of techniques with increasing sensitivity. Recent methods exploit the strong optical depopulation pumping of the initial atomic (ionic) state to detect the optical resonances by state-selective charge-changing collisions and ion (atom) counting instead of the less efficient photon counting [1-5]. The sensitivity limit is in many cases determined by the presence of strong beams of stable or long-lived isobars, superimposed on the weak beams of short-lived isotopes. One type of solution to this background problem would be the development of element selective (laser) ion sources, or very-high-resolution mass separators. We used radioactive detection of the optically pumped ions after stateselective charge exchange (later referred to as ROC) to avoid the influence of stable isobaric contaminants. In this Letter, we report the first application of this method to extend the calcium isotope shift measurements beyond the N =2S shell closure, thereby also providing the first experimentally determined charge radius for a short-lived isotope in the 20:< Z < 28, A^ > 28 mass region.The charge distribution of the isotopes in the calcium chain has been investigated extensively (see Ref.[6] and references therein), so far covering all isotopes from "^^Ca to "^^Ca, both being doubly magic nuclei. All existing laser spectroscopy data were calibrated with muonic isotope shifts combined with electron scattering data in a comprehensive analysis [6], yielding a parabolic dependence of the mean-square charge radii as a function of neutron number, with a pronounced odd-even staggering. Several Hartree-Fock-based calculations have been performed by Caurier and Poves [7], Waroquier, Heyde, and Wenes [8], and Barranco and Broglia [9], finally resulting in a good agreement with experiment. Both the parabolic trend and the odd-even staggering were also reproduced by Talmi [10] using a shell-model-based theory. An extension of the isotope shift and nuclear moment data across the A^ =28 shell closure is of fundamental interest as it provides a crucial test for Hartree-Fock calculations and for shell model descriptions far from stability.Nuclear spectroscopy experiments show, as expec...

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“…17, show that the trends and [81] (for even isotopes only). Solid circles: experimental data from [70] (for S n ) and [93][94][95] (for δ r 2 ch ).…”

Collinear laser spectroscopy at ISOLDE: new methods and highlights

“…These particle/radiation detection techniques require each a dedicated experimental setup at the collinear beam line, and each technique is only applicable on particular elements or isotopes. For example, optical pumping of neutral atoms, detected by state-selective collisional ionisation [6] and optical pumping of singly-charged ions, detected by state-selective neutralisation [7,8] are suitable techniques for the noble gasses (e.g. Ar isotopes) and alkaline earth elements (e.g.…”