Probing the Drip-Lines through Momentum Distributions
About a decade ago it was found that nuclei in the vicinity of the neutron drip-line under certain circumstances develop very extended valence neutron distributions, a phenomenon today known as halo states. The recent development of radioactive nuclear beams has provided us with a perfect tool for investigating these particular nuclei. There are several nuclei that exhibit halo structure such as 11Be, 19C (one-neutron halos), 6He, 11Li, 14Be, 17B (two-neutron halos), and 8He (four-neutron halo). Among their signatures, large total reaction and halo-removal cross-sections and narrow momentum distributions have been found. This thesis work will mainly deal with fragment momentum distributions detected in nuclear break-up of halos. We want to find answers to some fundamental questions like: What will actually happen when a loosely bound projectile collides with a target nucleus, and to what extent does the measured momentum of a break-up fragment reflect its internal motion inside the projectile?
The simple interpretation of the momentum distribution in form of a Fourier transform of the wave function has recently been questionable. The peripheral nature of the halo removal process and the final-state interaction in the exit channels are shown to play an important role in defining the final momentum distribution.
We have calculated particle momentum distributions from 8He fragmentation on a light target, utilizing an analytical five-body wave function for 8He. Without additional fitting parameters the experimental neutron and 6He transverse momentum distributions were reproduced, and the main ingredients in the fragmentation process were found to be the neutron motion in 8He, the neutron-target interaction, and the final-state interaction between the neutron and 6He through the 7He resonance.
Core longitudinal momentum distributions for one-nucleon halos have been investigated. The momentum distribution is shown to be determined mainly by the one-nucleon separation energy and the orbital angular momentum of the halo nucleon, and due to the reaction mechanism sensitive to the asymptotic part of the wave function only. Detailed calculations for the loosely bound carbon isotopes 17,19C and the one-proton halo 8B have been performed and compared to existing experimental data.