Precision moments of the $^{11}$Li halo nucleus

One of the actual key topics in nuclear physics is the study of exotic nuclei, i.e. nuclei with extreme properties, such as an extreme ratio of the number of neutrons and protons, excitation energy or total nuclear spin. The interest for exotic nuclei has increased enormously since the recent progress in nuclear accelerators. This made it possible to produce exotic nuclei with sufficient rates. The study of neutron rich nuclei revealed unexpected phenomena such as weakening of closed shells and the formation of neutron halos. These phenomena indicated that the interaction between the nucleons - the strong force - was not well enough understood. In particular the influence of extreme conditions on the spin-orbit term, which had given rise to the magic numbers in the shell model, was not understood. As a consequence, the parametrization of existing nuclear models such as the shell model had to be adapted and new models were developed. The magnetic and quadrupole moment of exotic nuclei are stringent tests for these new developed nuclear models. They contain a lot of information about the structure of the nuclear state: the magnetic dipole moment is sensitive to the orbitals of nucleons that are not paired off to zero spin. The electric quadrupole moment gives information on the deformation of the charge distribution of the nucleus. Of particular interest are the quadrupole moments of the ground states of the neutron rich $^{9}$Li and $^{11}$Li. $^{11}$Li is a drip line nucleus with a magic number of neutrons (eight) and is a textbook example of a halo nucleus. It consists of a $^{9}$Li core and two loosely bound neutrons. The influence of these two neutrons on the charge distribution of the core can be determined by Q($^{11}$Li)/Q($^{9}$Li). The expected influence is small. Therefore precision measurements are needed. In order to determine the quadrupole moments precisely via multiple-rf-NQR, first an accurate knowledge of the magnetic moments is needed. These magnetic and quadrupole moments were studied using the $\beta$-NMR/ NQR (Nuclear Magnetic Resonance/ Nuclear Quadrupole Resonance) technique at the ISOLDE facilities in CERN. For the production of the required spin-orientation, the technique of optical pumping was used. The measurements were performed in collaboration with the COLLAPS group of Mainz (Germany) lead by Prof. Dr. R. Neugart. This group had already measured these moments at the ISOLDE facilities in CERN, using optical pumping to create nuclear spin-orientation. But the obtained accuracy was not good enough to be conclusive about the influence of the two loosely bound neutrons. Therefore a precision measurement was performed with an improved version of the same set-up. Like this the present know-how was optimally used. For the optimization of the parameters of the set-up and the calibration of the magnetic field, the well produced $^{8}$Li isotopes were used. Both the parameters for the optical pumping set-up and those for the $\beta$-NMR/NQR set-up were optimized. My main contributions concerned the optimization of the $\beta$-NMR/NQR.