NeuLAND - FAIR

2. Physics Scenarios: Requirements for
NeuLAND
In the following sections we detail the demands for the detection of fast neutrons derived
from the main types of experiments within the broad R 3 B physics program. For a more
detailed description of the overall R 3 B physics programme we refer to the Conceptual
Design Report for FAIR [CDR-01] and the R 3 B Technical Proposal [R3B-05] and references
therein. NeuLAND will be located downstream from the target. In the full
acceptance mode, the distance from the target is chosen such, that the face-size of the
detector covers the same angular range as the yoke of the dipole magnet R 3 B-GLAD,
namely 80 mrad. For a detector of 2.5×2.5 m 2 face-size the full-acceptance distance is
15.5 m downstream from the target. For some applications, see e.g. section 2.2, the
highest possible resolution is demanded. Here we profit from the length of the R 3 B cave
allowing to position the detector up to 35 m downstream. As a third option NeuLAND,
or parts of NeuLAND, can be placed closer to the target for the detection of neutrons
stemming from quasi-free scattering interactions, see section 2.4.
2.1. Evolution of the Collective Response of Exotic Nuclei
Electromagnetic excitation of exotic nuclei is a very powerful tool for exploring the evolution
of collective phenomena like giant resonances, especially the giant dipole resonance
(GDR) as a function of neutron-proton asymmetry [Har-01]. At beam energies of up to
1000 AMeV, cross sections for dipole excitations are large, on the order of barn, and the
excitation energies transferred reach up to approximately 20 MeV [Ber-85]. The LAND
collaboration has played a pioneering role in this field discovering the double giant dipole
resonance in stable nuclei [Bor-03], and more recently the pygmy mode in neutron-rich
Sn isotopes [Adr-05]. The appearance of low-lying strength in exotic nuclei [Kli-07],
[Wie-09], is particularly interesting from a nuclear structure point of view since its occurrence
is interpreted as a consequence of neutron-proton asymmetry and the evolution
of skin effects. Similar to the collective giant resonances, this so-called pygmy strength
is related to bulk properties of nuclei and nuclear matter. Various, and partly contradictory,
microscopic theoretical models describe this resonance as discussed in an overview
by N. Paar et al. in Ref. [Paa-07] and references therein. It has been suggested, that the
density dependence of the symmetry energy close to saturation density is correlated with
the low-lying dipole strength in neutron-rich nuclei [Pie-06], very similar to the earlier
reported correlation of the symmetry energy parameters to the neutron-skin thickness in
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