NeuLAND - FAIR

decaying into 24 O plus 4 neutrons and the tetra-neutron system. In the next section we
summarize the capabilities and design goals of NeuLAND. In chapter 2 we provide a more
detailed view of the various physics cases of R 3 B with emphasis on those cases where
the performance of the newly designed NeuLAND is key to crossing borders into new
frontiers, i.e., entering ”Neuland” (terra incognita) in physics with radioactive beams.
1.2. NeuLAND Design Goals – General Remarks
The detection of fast neutrons plays a crucial role for the exploration of light and heavy
exotic nuclei towards the neutron dripline. Since the neutron separation thresholds decrease
with increasing neutron-proton asymmetry, the emission of several neutrons after
excitation is the dominant decay process. Neutrons emitted from projectiles with high
velocities in the laboratory frame are kinematically strongly forward focussed, although
isotropically emitted in the projectile rest frame. This allows for a very efficient coverage
of the full solid angle with moderate detector sizes. The excitation energy of the
projectile can be determined, using the invariant-mass technique, from measurements of
the momenta of all emitted particles. The neutron momenta are determined from the
measured position of the interaction in the neutron detector and the time-of-flight from
the target to the neutron detector. The desired resolutions for momenta and thus the
excitation energies, see details in chapter 2, lead to the required spatial resolutions of
σx,y,z ≤ 1.5 cm and a time resolution of σt ≤ 150 ps. The main design goals comprise,
besides the improved resolution, a detection efficiency above 95% for single neutrons
for a wide energy range, a large geometrical acceptance and an excellent multi-neutron
hit reconstruction efficiency. The major achievements in performance, compared to the
present LAND detector, are an improved resolution by a factor of three, the extension of
the very high efficiency for neutrons down to lower energies, and the substantial improvement
of the multi-neutron reconstruction efficiency from a few percent up to 60 percent.
The latter will be a major and necessary advantage for future studies with more neutronrich
systems including nuclear states beyond the neutron dripline. The performance of
the detector is described in great detail in this report on the basis of extensive simulations
and prototype tests. Here, we give a brief overview on the main features with some
examples including large acceptance, high resolution, a large multi-neutron-hit resolving
power, and high efficiency in a very wide energy window.
The active area of the detector of 250 × 250 cm 2 allows to cover the full acceptance
of ±80 mrad determined by the gap of the dipole magnet if the detector is placed
15.5 m downstream of the target. This solid-angle coverage corresponds to a 100%
acceptance (4π) measurement of neutrons with kinetic energies up to around 5 MeV in
the center-of-mass (CM) frame if emitted from a projectile fragment with a kinetic energy
of 600 AMeV. This has to be compared with a typical neutron energy distribution, e.g.,
a Maxwellian distribution of a statistical decay, which peaks at kinetic energies of 1 to
2 MeV and contains more than 90% of the events within the 5 MeV coverage. At higher
beam energies, either a larger acceptance can be chosen (up to about 10 MeV neutron
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