Nuclear structure studies of rare francium isotopes using Collinear Resonance Ionization Spectroscopy (CRIS)

It was known for many years that nuclei possessing certain numbers of protons (Z) and neutrons (N), called the magic numbers (8,20,28,50,82,126...), exhibit characteristic behavior and are in general more stable than their neighboring isotopes. As the capabilities of producing isotopes with more extreme values of Z and N increased, it was realized that those spherical nuclei only represent a small fraction of the total number of isotopes and that most isotopes are deformed. In order to study exotic isotopes and their deformation, it was necessary to develop new experimental techniques that would be powerful enough to be able to cope with very small production yields, but precise enough to measure the nuclear properties (such as radii and moments) with relatively small uncertainties. One technique that can measure nuclear properties of scarcely produced isotopes is in-source resonant ionization, but this technique does not allow for sufficient precision to deduce nuclear quadrupole moments. Furthermore, this technique can only be applied to isotopes that are not surface- ionized, and is therefore not applicable for alkaline elements. On the other hand, a technique that can precisely measure atomic hyperfine structures and therefore precisely extract nuclear properties such as nuclear spin, magnetic dipole and electric quadrupole moments and mean-square charge radii is collinear laser spectroscopy with fluorescence detection. This technique however lacks the efficiency needed to study very exotic species. The aim of this PhD work was to apply a new technique which would be both efficient and precise: Collinear Resonance Ionization Spectroscopy (CRIS), to the study of the rare francium isotopes. The interest in studying the francium (Z = 87) isotopes lies in the fact that its neutron-rich isotopes (around mass A = 225) are located in a region of the nuclear chart that exhibits a rare form of deformation: octupole deformation. Such type of deformation leads to reflection-asymmetric nuclear shapes (pear shapes). Previously the 220−228Fr isotopes were studied with collinear laser spectroscopy with fluorescence detection, which showed that the charge radii of these isotopes exhibited unusual behavior. This stimulated decay spectroscopy studies to be performed on these francium isotopes, which showed that they exhibit several signatures of reflection asymmetry. The goal of this work was to extend the laser spectroscopy measurements to either side of this region, to determine the range in which these octupole deformations occur. Additionally, the francium isotopes with less than the magic N = 126 neutrons, were known to exhibit a lowering of the proton 3s−1 intruder state as more neutrons are 1/ 2 removed, and it was the goal of these experimental campaigns to push the limits of the studied francium isotopes towards 199Fr, where this state is predicted to be the ground state. The experiments were performed in two periods. The first two experimental campaigns in August and October 2012 at ISOLDE, CERN, measured for the first time the magnetic dipole moments and changes in mean-square charge radii of the 202−206Fr (and their isomers) and the neutron rich 218m,219,229,231Fr isotopes. The measurement of the hyperfine structure of 202Fr, produced by less than 100 particles/s, was made possible due to the demonstrated > 1% high efficiency of the CRIS technique. The high resolution was not attained however, as it was limited by the 1.50(5) GHz linewidth of the used laser system. The measured magnetic dipole moments showed that the odd proton occupies the 1h9/2 shell model orbit in 219Fr, while in 229,231Fr it occupies the deformed Nilsson 1/2[400] orbital, originating from the (3s1/2)−1 intruder state. The mean-square charge radii results agreed with 220Fr being a transitional isotope exhibiting weak octupole correlations, while 228Fr lies outside of the region of reflection-asymmetric shapes. Following these experiments, technical developments were performed as part of this work on the CRIS off-line ion source, leading to the possibility of providing stable beams of potassium. These beams were used to test the new laser scheme consisting of a narrowband continuous wave laser which was chopped by use of a Pockels cell, with this pulse being separated in time from the ionization laser pulse for reducing line broadening effects. This enabled the CRIS technique to reach the goal of high resolution, exemplified by the obtained 20(1) MHz wide hyperfine structure peaks of francium during a run in November 2014. The very well resolved hyperfine spectra allowed the extraction of the spectroscopic quadrupole moment of 219Fr. This value showed that the motion of the odd proton is decoupled from the deformation-axis of the nucleus, as was the case in 221Fr as well. This experiment showed that the CRIS technique is able to achieve high resolution without sacrificing efficiency, which has opened the door to many studies of exotic nuclei using the technique.