Excursions Along the Drip Line

Doktorsavhandling, 1995

The drip lines are the outer boundaries of the chart of the nuclides, terminating the isobaric chains on both sides of the valley of .beta.-stability with respect to stability against particle emission. The neutron drip line is, at the moment, only experimentally accessible for the very lightest elements. When approaching the neutron drip line, the structure of the nuclei starts to show anomalies compared to stable nuclei. The best known case is 11Li, having an extremely dilute structure with two loosely bound neutrons surrounding a 9Li core with normal density. This kind of configuration has been labelled a neutron halo and can be found in several nuclei along the drip line. This work regards experimental studies of drip-line nuclei through dissociation reactions at high beam energies and spectroscopy of .beta.-delayed particles. The effect of cross-talk between neutron detectors has been measured and compared to the results of a simulation code. The measurements confirmed the validity of the code and showed a negligible contribution of cross-talk for the neutron-detector arrays used in experiments at GANIL. The .beta.-delayed charged particles following the precursors 6,8He have been studied at ISOLDE, CERN with a newly developed gas-detector telescope. The branching ratios for .beta.-delayed deuteron emission from 6He and .beta.-delayed triton emission from 8He were measured with improved accuracy and decreased low-energy cut-off compared to earlier experiments. In an experiment at GSI, dissociation of 8He, 11Li and 11Be, with energies of 240 MeV/u, 280 MeV/u and 460 MeV/u respectively, was studied. From the experimental data, cross sections and momentum distributions could be extracted. The reaction mechanisms could be probed by studying various reaction channels, revealing an invariability regarding the width of the neutron momentum distribution when concentrating upon the core-breakup reactions. The role of the unbound resonance 10Li has been elucidated by population through one-neutron stripping from 11Li and one-proton stripping from 11Be. The longitudinal and transverse momentum distributions from the reaction 11Li -> 9Li + X have been measured at the same beam energy. The widths were found to be identical, and the lack of Coulomb broadening in the transverse case indicates that the halo-removal reactions predominantly take place at large impact parameters.