NeuLAND - FAIR

2.4. Quasi-Free Scattering
The measurement of quasi-free nucleon-knockout reactions constitutes a quantitative
tool to provide information on the single-particle structure of nuclei. From electroninduced
proton-knockout reactions, spectral functions and momentum distributions of
nucleons inside the nucleus have been deduced. Such measurements revealed the role
of correlations which govern the distribution of the single-particle strength at the Fermi
surface. Beyond the shell-model-like correlations among valence nucleons and their coupling
to collective states, nucleon-nucleon correlations have been identified, which lead
to the highest components of the nucleon momentum distribution and to a depletion of
occupancy of single-particle states. From such measurements, it has been concluded that
about 20% reduction of spectroscopic factors should be attributed to such short-range
correlations. In total, a typical occupancy of single-particle states in the order of 60%
has been established. Recent (e,e’p) experiments performed at the JLAB have measured
proton knockout reactions at high momentum transfer and detected a second correlated
nucleon in coincidence [Sub-08]. The authors concluded from such measurements, that
the nucleon-nucleon correlations inside a nucleus causing the high-momentum components
of nucleons are related mainly to neutron-proton pairs, and to a lower degree to
p-p or n-n pairs. This conclusion is, however, not yet established. It would of course be
extremely interesting to probe such correlations for neutron-proton asymmetric nuclei,
where a change of the importance of the different types of correlations is expected, as it
is for asymmetric nuclear matter. The amount of correlations in neutron-rich matter, for
instance, has significant influence on the properties of neutron stars. Radioactive beams,
potentially, provide the possibility to probe the role of nucleon-nucleon correlations in
nuclei as a function of neutron-proton asymmetry.
Nucleon-knockout reactions from high-energy radioactive beams using light targets (usually
Be or C) have been extensively used in the past decade to probe the shell-structure of
exotic nuclei. It has been demonstrated that this reaction is well understood by Eikonal
reaction theory and that spectroscopic factors for valence nucleons can be deduced with
rather good accuracy and less ambiguities as it is the case, for instance, for transfer reactions
at lower beam energies. The method has recently also been applied to knockout
of more deeply bound nucleons, and a strongly reduced spectroscopic factor has been
found compared to shell model calculations. Gade et al. have published a systematic
investigation [Gad-08], where they plot the ratio of experimental-to-theoretical singleparticle
cross sections as a function of the difference of separation energies of neutrons
and protons, i.e., the difference in the Fermi energies. The surprising result is, that proton
knockout from a neutron-rich nucleus yields to a much larger reduction of this ratio,
i.e., a ratio of only 0.3. The same holds for neutron knockout from a neutron-deficient
nucleus, while the factor approaches unity for weakly-bound nucleons, and about 0.6 for
symmetric cases, as established from (e,e’p) experiments. This has raised the question
of isospin effects on the correlations, but also a debate on the experimental method, observable,
and their interpretation. In contrast to an electron-induced knockout reaction,
the reaction with a beryllium or carbon target leads to a localization of the reaction
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