Neutron-rich polonium isotopes studied with in-source laser spectroscopy

This work studies the unknown region of neutron rich polonium isotopes. The polonium isotopes, with Z=84, lie above the magic lead nuclei (Z=82). The motivation for this research can mainly be found in these lead nuclei. When looking at the changes in the mean square charge radii beyond the N=126 shell gap, a kink is observed. This kink is also found in the radon (Z=86) and radium (Z=88) isotopes. The observed effect cannot be reproduced with our current models. The polonium isotopes yield more information on the kink and they are also able to link the known charge radii in lead isotopes to those in radon and radium. Additionally, the nuclear moments of the odd-neutron isotope $^{211}$Po are investigated. This nucleus has two protons and one neutron more than the doubly magic nucleus $^{208}$Pb. Nuclear moments of isotopes close to this doubly magic nucleus are good tests for the theoretic models. Besides pushing the models to their limits, the nuclear moments of $^{211}$Po also yield new information on the fairly unknown region beyond N=126. In-source laser spectroscopy of many polonium isotopes ($^{196,202,206−211,216,218}$Po) was performed at the ISOLDE facility of CERN. The use of a laser with a high resolution (2-5 GHz) makes it possible to scan the hyperfine structure of the electron cloud. This structure holds the information on the nuclear properties. The laser spectra were created by recording the amount of ions being produced as a function of the used laser frequency. The detection of these ions was dependent on the half-life and decay mode of the isotope being studied. The main detection setup for the neutron-rich polonium isotopes was the Windmill system. Here, the ions were implanted in carbon foils and their subsequent $\alpha$-decay was detected. Additionally, a Faraday cup and the ISOLDE tape station was used to measure the longer lived or $\eta$- decaying isotopes respectively. The analysis started from the $\alpha$- spectra obtained at the Windmill setup. Identification of the $\alpha$-peaks was needed to create the laser spectra. The profile of the laser spectra was studied in detail in order to determine what the best method is to analyze these spectra. The starting point was a Voigt profile. A Voigt profile consists of a Gaussian, from the temperature of the ion source, convoluted with a Lorentzian profile, coming from the lasers. In order to accurately extract the centroid positions of the resonances, the asymmetry present in the resonance profiles needed to be taken into accounted. This was done with two different methods, which were compared at the end of the analysis. The knowledge obtained from the spectra of the even isotopes was used to gain more control on the analysis of the odd isotopes. These spectra are more complex because of the magnetic and quadrupole interaction which need to be taken into account. The King plot was the final part of the analysis. In order to extract the changes in the mean square charge radii from the isotope shifts, one needs to know the field shift factor F and the specific mass shift factor KSMS. These can be calculated using large-scale atomic calculations. The King plot was used to take into account the discrepancies between the data and the calculations. The final chapter of this work contains the extracted changes in the mean square charge radii and the nuclear moments of $^{211}$Po. The magnetic moment (−1.196(58)$\mu$N) lies close to the calculated Schmidt moment using effective g-factors (−1.339$\mu$N ). This shows that the expected ground state $((\pi 1h_{9/2})^{2}, \nu2g_{9/2})$ is the main contribution to the nuclear wave function. The electric quadrupole moment (-69(11)efm$^{2}$) is more difficult to assess because of different issues surrounding its extraction and the overall lack of knowledge in this region. The changes in the mean square charge radii are best viewed together with the neighboring elements. Here it was seen that the slope of the kink increases with the proton number Z. This type of systematic behavior is needed to further understand this shell effect.