Frequency-Comb based laser spectroscopy of Halo Nuclei: The nuclear charge radius of 11Be

Nuclear charge radii of halo isotopes provide a direct proof of the halo structure of neutron rich isotopes. If they are compared to the matter radii from nuclear reactions, a geometrical picture of the two- and three body structures can be extracted. We have determined the nuclear charge radii of 7,9,10,11Be by high-precision laser spectroscopy [1,2] and combination with accurate calculations of the mass-dependent isotope shifts [3,4]. The on-line measurements were performed with collinear laser spectroscopy in the 2s1/2 → 2p1/2 transition on a beam of Be+ ions. Collinear and anti collinear laser beams were used simultaneously and absolute frequency determination was applied using a frequency comb. This yielded an accuracy in the isotope-shift measurements of about 1 MHz. The charge radius of the beryllium was found to decrease from 7Be to 10Be and then increases for the halo nucleus 11Be. We compare our results with predictions of ah initio nuclear structure calculations and the conclusions about the geometric size of the halo will be discussed.(author)