NeuLAND - FAIR

GEANT3 interface. Here, we compare data from a deuteron breakup experiment, performed
in the year 1992, with simulation results. Various deuteron beam energies of 170,
270, 470, 600, 800, and 1050 AMeV were used, thus the characteristics of the detector
can be determined for a wide range of neutron energies.
During the interaction of the primary neutrons secondary particles are produced. For
the energy deposited in the scintillator, secondary protons and neutrons dominate but
also gamma-rays and electrons contribute, especially at small energies. The response of
the detector strongly depends on the choice of the threshold for the light detection of
the photomultipliers.
Light quenching for protons was taken into account using Birk’s relation [Bir-64]
L(∆E) = ∆E/(1. + 0.013 ∗ dE/dx + 9.6E − 6 ∗ (dE/dx) 2 ), (4.1)
where L is the light output, ∆E the energy deposit in the scintillator in MeV, and dE/dx
the energy loss in MeV/g/cm 2 .
A typical interaction of a high energy neutron with an iron nucleus produces several
protons, neutrons and gamma-rays. If the protons have sufficient energy to leave the
iron sheet, they deposit energy in a scintillator, giving rise to scintillation light. The
neutrons travel through the detector and produce further secondary particles. This
happens either again in iron sheets or directly in the scintillator material by elastic
scattering on protons. The gamma-rays produce electrons which again deposit energy in
the scintillators. Therefore, in a typical LAND event several modules fire. The detected
multiplicity strongly depends on the photomultiplier threshold applied. Figure 4.4 shows
tracks of typical events for 470 MeV neutrons.
Sometimes, no light is produced during the first interaction of the primary neutron.
This leads to an incorrect determination of the position and consequently to an incorrect
neutron momentum via the ToF method. Figure 4.5 shows the distance between the first
interaction points of 470 MeV neutrons and the position of the first light generation. In
this example 40 % of the events are outside of the geometrical dimension (10 cm) of a
module.
A large detection efficiency is a key feature of LAND. The simulated efficiencies in
table 4.2 were determined with the GEANT3 and GEANT4 simulation packages and
are compared with the measured values.
The most important quantities for the comparison between data and simulation are the
multiplicity of modules, the total deposited energy, the energy deposit during the first
interaction and the energy deposit in a single paddle. Figure 4.6 presents simulation
results for GEANT3 in comparison with the measured data. Thresholds were adapted
within reasonable range to match the multiplicity spectra. An all-over factor of 0.4
necessary to adapt the GEANT3 total energy findings to the measured energy deposits
points to some mismatch in the absolute calibration of the experimental data. With
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