NeuLAND - FAIR

investigated. Three values for the time resolution were simulated: σt=0 ps, 100 ps,
and 150 ps. For distances of 15.5 m, the ultimate relative energy resolution has the
highest demands on time and position resolution. For none of the investigated bar cross
sections the relative energy resolution of ≤20 keV is obtained with a time resolution of
σt=100 ps. Only a detector with a scintillator bar cross section of 3 × 3 cm 2 and a time
resolution of σt=50 ps fulfills the demanded relative energy resolution of ≤20 keV. For a
target-detector distance of 35 m, the maximum achievable distance between the target
and NeuLAND in the R 3 B experimental hall, scintillator bars with a cross section of
5×5 cm 2 and a time resolution of σt=150 ps are sufficient to meet the design criterion.
Again, we compare the results from the phase-space simulations for one example to the
result of R 3 BRoot simulations. The influence of time resolution is shown in figure 4.9
for a distance of 35 m and a cross section of the scintillator bars of 5 × 5 cm 2 . The
values of the two simulations are in good agreement. A relative energy resolution of
σ(Erel)=15 keV for σt=150 ps from R 3 BRoot, similar to the result of σ(Erel)=17 keV
derived within the phase-space approach.
However, to cover the acceptance of 80 mrad defined by the magnet yoke in this highresolution
mode at 35 m would require an unreasonable large face-size of NeuLAND of
more than 5.6 m. For the detector design we finally select scintillator bars with a cross
section of 5 × 5 cm 2 , an active length of 250 cm, and a corresponding 250 × 250 cm 2
face-size. Thus, at a distance of 35 m the NeuLAND acceptance amounts to about
35 mrad. We note, that in physics cases, where the high-resolution mode is demanded,
this angular coverage is fully satisfactory, since small relative energies are investigated,
which are characterized by a small relative emission angle between neutron and projectile
fragment. The full-acceptance for the final detector face-size is given at a distance of
15.5 m to the target.
The use of larger bar cross sections together with larger face-sizes and volumes, assuming
a fixed number of modules, has several advantages. First, the demands for time resolution
can obviously be relaxed, since the full-acceptance is covered at a larger distance. For a
detector with a time resolution of σt=100 ps at 12.5 m from the target (full-acceptance
for 200×200 m 2 ) we derive, for example, a similar relative energy resolution as for a
detector with σt=150 ps at a distance of 15.5 m. Second, and this turns out only on
a later stage of simulations, see section 4.5, the larger detector volume is of advantage
for the association of hits from the secondary particles in the detector to the primary
neutrons, and thus for the multi-neutron recognition. The overlap of shower volumes
of individual neutrons at larger distances is smaller. Also, since calorimetric properties
play an important role in the multi-neutron recognition, the larger detector volume is
advantageous, because of a better collection of energy deposited by secondary particles.
From the considerations presented in this chapter and from the prototype results, see
chapter 3, we select scintillator bars with active sizes of 250 × 5 × 5 cm 3 as NeuLAND
submodules. The submodules in the standard configuration will be arranged in planes
with face-sizes of 250 × 250 cm 2 .
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