Laser Spectroscopy of the Boron Isotopic Chain

Abstract

The charge radius of a nucleus is a fundamental physical property that corresponds to the binding strength and the structure of the bound nuclear system. This observable is particularly important for the investigation of so-called halo nuclei, which consist of a compact nuclear core with typical nuclear density and a dilute cloud of halo nucleons. Neutron halo nuclei have been subject to numerous investigations, but little is known about proton-halo systems. The isotope 8B is believed to be a prototype of a proton-halo, which was concluded indirectly from measurements of its quadrupole moment and analysis of the momentum distribution of breakup fragments. High-precision laser spectroscopy can provide direct proof of the halo structure by measuring the nuclear charge radius. Such measurements were performed previously for the most prominent (neutron) halo isotopes up to Z = 4 (beryllium). Extending these investigations to the boron isotopes (Z = 5) is the subject of this thesis. It covers the design and installation of the experimental setup for the on-line measurement of the short-lived 8B (t1/2 = 770 ms) as well as first off-line measurements of the charge radius difference between the two stable isotopes 10B and 11B. The halo-candidate 8B is produced in a 3He(6 Li,8B)n reaction in inverse kinematics and the energetic 8B ions are stopped in a gas cell and then transported to the collinear beamline by radiofrequency ion guides. The production of 8B was optimized by improving the cryogenic gas target to avoid saturation effects. It was observed that the 8B ions that leave the gas cell have water molecules attached. While this improves the transport process, it prohibits laser spectroscopy on the atomic system. Therefore, a molecular breakup station based on the transmission of the molecules through nanometer-thin carbon foils was designed, built and tested. The apparatus to perform collinear laser spectroscopy was also installed at the experimental site during this work and the performance of its fluorescence detection region has been significantly improved. The progress achieved represents a significant step towards laser spectroscopy on the exotic proton-halo candidate 8B in the near future. The isotope shift between the stable isotopes of boron 10B and 11B was measured on a collimated atomic beam. Beams of two lasers crossed the atomic beam in perpendicular alignment to perform resonance ionization mass spectrometry. The first laser excited the 2p → 3s ground state transition and the second laser was used for non-resonant ionization of the excited atoms. Exact control of the overlap angle of laser and the atomic beam is essential to eliminate the first-order Doppler effect, which otherwise introduces significant uncertainties for the spectroscopy of these light ions. Therefore, an elaborated double-pass scheme has been used where the uncertainty in the overlap angle was minimized together with other sources of systematic uncertainties that are typically prevalent in laser spectroscopy. The isotope shift between 10B and 11B was measured for the first time with sufficient accuracy to extract the difference in the mean-square nuclear charge radius. Results are compared with predictions from state-of-the-art ab-initio nuclear structure theories and show reasonable agreement with the no-core shell model as well as Green’s function Monte Carlo calculations.