GSI and FAIR

What are Exotic Nuclei?

Exotic Nuclei are nuclei with an extraordinary ratio of protons and neutrons. Typically, they are (highly) unstable and they decay into more stable nuclei. They can either be produced in Compound-Nucleus reactions where two nuclei are fused at rather low energies, or they can be made by projectile fragmentation at medium energies. Since Exotic Nuclei tend to decay , beams of these nuclei are called radioactive. The picture on the right hand side shows the number of protons /*Z*/ versus the number of neutrons /*N*/ for the known nuclei. The binding energy of each nucleus is color-coded with the lighter color indicating less bound nuclei. The region of possibly synthesisable nuclei extends both to the proton-rich as well as the neutron-rich side.

Up to now, the standard model of nuclear structure - the Nuclear Shell Model - has only been tested in regions close to the valley of stability in the nuclear chart. While leaving this region of stable nuclei new phenomena are expected and new insights into the complex nuclear many-body problem should be obtainable. The detailed understanding of the structure of Exotic Nuclei is intimately connected to astrophysics, since the nucleosynthesis - which usually happens inside massive stars - is typically taking place in regions of extreme neutron to proton ratios.

Experimental Activities:

• Decay of Proton-Rich Nuclei

• Coulomb Dissociation of B-8 and the Solar Neutrino Flux

These experiments are mainly motivated by the surprisingly low detected flux of neutrinos emitted from the sun. The discrepancy between the prediction of the standard solar model combined with the standard model of elementary particles is a factor of two to three. One reason could be that one crucial ingredient of these models - the fusion reaction cross section for Be-7 + gamma is not known precisely and direct measurements tend to be extremely difficult due to the low cross section. Therefore, we have started to determine this cross section by the reverse reaction: the dissociation of B-8 in the (virtual) photon field of a heavy nucleus as it is depicted on the figure at the left hand side. Due to the motion of the beam relative to the target nucleus Pb-208 the electrostatic Coulomb field is Lorentz-compressed and the field of (virtual) photons becomes quite dense leading to the breakup of B-8. The experiment took place at the Heavy Ion Synchrotron SIS at GSI in Darmstadt/Germany. We used the Fragment Separator to produce the radioactive beam of B-8 which then was delivered to the Kaon Spectrometer KaoS where the breakup took place. The setup of the Kaon Spectrometer is shown on the right. The particle enters from the left and hits the target after its position has been measured by two Parallel Plate Avalanche Chambers (PPAC). At this point, some of the nuclei break up in the target and their relative angle is determined by four Silicon strip detectors (Si1-2 and Si3-4). The energy of the decay particles is then measured by the magnetic spectrometer consisting of two deflecting magnets (quadrupole and dipole magnet) and two large area Multi-Wire Proportional Chambers (MWPC-L and MWPC-N) and a wall of scintillation detectors (F-PED). Several publications can be found here.