NeuLAND - FAIR

e studied at R 3 B at FAIR include nuclei which are produced in explosive astrophysical
scenarios where the heavy elements are synthesized in the r-process. While ground-state
properties like masses and half-lives are the most important properties necessary for
modeling the rapid neutron capture process in an equilibrium stage, it has been shown
that for an accurate theoretical description of the r-process the dipole response of these
nuclei plays an essential role in determining the final abundance of nuclei as observed
in the solar system. A systematic change of the response of nuclei alters the result of
the elemental synthesis significantly [Gor-98], [Rau-08]. An experimental and theoretical
effort has to be undertaken in order to understand the response of these neutron-rich
nuclei. Since the neutron separation energies for these nuclei are typically approximately
one to two MeV only, the decay of excited projectiles involves many neutrons. In order
to enable an invariant-mass analysis, the momentum vectors of the different neutrons
have to be resolved. Typically around four neutrons from the decay of the GDR will
have to be detected. NeuLAND will be the first detector which will be capable to satisfy
such a demand. An efficiency for a correct detection of a four-neutron event of about
60% is envisaged.
2.2. Dipole Strength at the Particle Threshold
Electromagnetic excitation of exotic nuclei is of importance as well from an astrophysical
point of view as mentioned already in the previous section. While the paths of the nucleosynthesis
processes like the r-, and rp-processes are essentially determined by nuclear
masses and lifetimes, the final abundance patterns depend also on the (p,γ) and (n,γ)
reaction rates. For the modeling of astrophysical processes with network calculations,
the reaction rates are usually calculated using a standard dipole response of nuclei as an
input to a Hauser-Feshbach calculation. It has been shown by Litvinova et al. [Lit-09]
that for a microscopically calculated dipole response (which predicts a significant shift
of strength towards lower excitation energies for neutron-rich nuclei) used as input, the
capture cross sections in the astrophysical temperature range change significantly, and
the final abundances as a consequence as well. Beside a fundamental understanding of
the dipole response and its decay patterns, a direct measurement of critical capture cross
sections is needed [Sur-09]. The heavy-ion induced electromagnetic excitation process
enables a measurement of the inverse process, i.e., the Coulomb breakup into fragment
and proton or neutron. The capture cross section or astrophysical S-factor can then
be deduced by using the detailed balance theorem. Prominent examples from previous
studies are the S-factor for the 7 Be proton capture [Sch-03], [Sch-06], the 14 C neutron
capture studied in 15 C Coulomb breakup [Dat-03], or the breakup of 7 Li [Ham-10].
The energy region where the cross section is needed is determined by the relevant temperatures
in the astrophysical processes. Usually, this corresponds to an energy window
around or even below 100 keV. For the measurement of the inverse process, this translates
into low relative kinetic energies between fragment and neutron in the CM system
after excitation. The excitation and detection process takes advantage of the high beam
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