In-Source laser ionization spectroscopy of 181,182Hg

The mean square charge radii of the mercury isotopes exhibit a large odd-even staggering for the neutron-deficient mercury isotopes, which is suggested to be a manifestation of shape coexistence. Shape coexistence is the phenomenon in which two (or more) distinct types of deformation occur at low energy in one and the same atomic nucleus. Understanding the occurrence of shape coexistence in atomic nuclei is a challenge faced by theories of nuclear structure. The investigation of the neutron-deficient mercury isotopes (Z = 80) plays an important role in the understanding of shape coexistence in this region of the nuclear chart. The theoretical description of shape coexistence is discussed in chapter 1. In a recent laser-spectroscopy experiment performed at the ISOLDE facility in CERN, the hyperfine spectra were measured for 15 different mercury isotopes ranging from $^{177}$Hg to $^{208}$Hg. One of the main goals of this experiment was to extend the currently existing knowledge of the mean square charge radius behavior down towards the more neutron-deficient isotopes and verify if the mercury isotopes become spherical again or whether the more neutron-deficient nuclei become even more deformed. In this work the difference in mean square charge radii between $^{181,182}$Hg and $^{198}$Hg are determined and compared with the literature values to verify consistency. The mercury isotopes were produced via a spallation reaction induced by a collision of accelerated protons with a molten lead target. The mercury isotopes were ionized using a three-step laser ionization process after which they were mass separated and send to a detection system. $^{198}$Hg was detected using a Faraday cup or the Multi-reflection Time-Of-Flight set-up with a multichannel-plate ion detector, while the $^{181,182}$Hg isotopes were sent to the Windmill system. In this set-up the isotopes are implanted in carbon foils and the alpha decay was detected using silicon detectors. The aim of the experiment was to construct the hyperfine spectra of which the isotope shift and change in mean square charge radius can be determined, as explained in chapter 2. The experimental set-up is explained in chapter 3. The final results are discussed in chapter 5. A key element in the laser spectroscopy measurements is the precise determination of the laser wavelength. However, during the past year a number of problems were identified in two wavelength meter models that are widely used at several laser laboratories. As one of the wavelength meters was of the same type as the one used for the mercury laser spectroscopy measurements, a series of systematic measurements to characterize the performance of the wavelength meters were performed in the In-Gas Laser Ionization and Spectroscopy (IGLIS) laser laboratory. The discrepancy has been resolved and it can be concluded that the charge radii measured with the in-source laser ionization spectroscopy technique are reliable.