Research program of the Reactions with Exotic Nuclei Group
Heavy-Ion Induced Electromagnetic Excitation at Relativistic Energies
The process of electromagnetic excitation in peripheral relativistic heavy-ion collisions is used to study the dipole response of exotic nuclei. The collective multipole response of nuclei is governed by bulk properties of nuclei and nuclear matter including the size and shape of the nucleus, the compressibility of nuclear matter, the symmetry energy, as well as the in-medium nucleon-nucleon interaction. For stable nuclei, the continuum response is dominated by the various giant resonances. Giant resonances are collective nuclear excitation modes exhausting a large fraction of the respective sum rules. The response of exotic nuclei with very asymmetric neutron-to-proton ratios differs considerable from the one known for stable nuclei. A redistribution of dipole strength towards lower excitation energies has been observed for neutron-rich nuclei exhibiting a resonance-like concentration of dipole strength close to the threshold, which is theoretically interpreted as a dipole vibration mode, where the more loosely bound valence neutrons oscillate against a core, often called Pygmy Dipole Resonance.
Nuclei at the neutron dripline exhibit a very particular dipole response. The loosely bound valence neutrons decouple from the rest of the nucleus (the core) and give rise to a characteristic dipole response resulting in a huge E1 transition strength directly at the separation threshold. The measurement of the E1 strength yields detailed information on structure of these loosely bound (halo) nuclei and is directly related to the quantum numbers of the single-particle states and the spatial extension of the wave function. In case of multi-neutron configuration correlations among the halo neutrons can be studied as well.
One-nucleon knockout reactions at high beam energies can be considered as a fast and sudden process. The measurement of the momentum and excitation energy of the residual fragment thereby allows extracting information on the single-particle properties of the knocked nucleon in the projectile. The change of the shell structure of nuclei when going away from the valley of stability is investigated using this reaction. In case of halo nuclei, this method is complementary to the above mentioned Coulomb breakup reaction. The knockout of protons from neutron-dripline nuclei populates unbound states beyond the dripline, the most neutron-rich systems we can think of. A kinematical complete measurement of all reaction products in the final states allows determination of the resonance energy (mass) of these systems as well as determining there structure.
Quasi-Free Nucleon Knockout in INverse Kinematics
Quasi-free nucleon knockout reactions like (p,2p) at high energies have been used in the past to study the single-particle structure of stable nuclei. We are developing an experimental method to utilize this reaction type in inverse kinematics with hydrogen targets and a kinematical complete measurement of the heavy residue after knockout. Aim is to investigate the single-particle structure and the role of (long-range, tensor, and short-range) nucleon-nucleon correlations in very asymmetric nuclei. (p,2p) as well as (p,pn) reactions are used. The cluster structure can be studied as well in (p,p alpha) reactions. (p,2p) reactions at the neutron dripline are being used as stepping stones across the dripline allowing a detailed investigation of the properties of nuclear systems with the most extreme neutron-to-proton ratios.
Results of the above-described research activities are documented in publications.