Laser systems for collinear spectroscopy and the charge radius of 12Be

Collinear laser spectroscopy has been used to investigate the nuclear charge radii of shortlived medium- and heavy-Z nuclei for more than three decades. But it became only recently be applicable to low-Z nuclei. This region of the nuclear chart attracts attention because so-called ab-initio nuclear models, based on realistic nucleon-nucleon potentials, can only be applied to the lightest elements due to the rapidly increasing calculational demands with the number of nucleons. Furthermore, strong clusterization of atomic nuclei occurs and the encountered halo nuclei are presently subject of intense research. The isotopic chain of beryllium exhibits the prime example of a one-neutron halo nucleus, 11Be, and the two- or four-neutron halo nucleus 14Be. 12Be is a key isotope between these two exotic nuclei and particularly interesting because the nuclear shell model predicts a shell closure for the magic neutron number N = 8. In the course of this thesis, several frequency-stabilized laser systems for collinear laser spectroscopy have been developed. At TRIGA-SPEC a frequency-doubled diode laser system with a tapered amplifier and a frequency comb-stabilized titanium-sapphire laser with a frequency doubling stage are now available for the spectroscopy of refractory metals above molybdenum. They have already been used for test-experiments and commissioning of the TRIGA-LASER beamline. Furthermore, frequency-quadrupling of the Ti:Sa laser was demonstrated to expand the emitted wavelengths into the 200 nm region. At ISOLDE/CERN a frequency comb-stabilized and an iodine-stabilized dye laser were installed and applied for laser spectroscopy of 9,10,11,12Be+. The improved laser system and the development of a delayed photon-ion coincidence detection improved the sensitivity of the beryllium spectroscopy by more than two orders of magnitude and, thus, the previous measurements of 7-11Be could be extended for the first time to the short-lived isotope 12Be. In addition, the accuracy of the absolute transition frequencies and of the isotope shifts of 9,10,11Be were significantly improved. Comparing the extracted charge radii with results of the Fermionic Molecular Dynamics model, the trend of the charge radii of the lighter isotopes can be explained by the pronounced cluster structure of the beryllium nuclei. Further it was derived that the ground-state wavefunction is clearly dominated by an intruder (sd)2 configuration. This is contradictory to the nuclear shell model expectation of a p2 ground-state configuration and strongly supports the previously observed breakdown of the N=8 magic shell closure in the beryllium isotope 12Be.