NeuLAND - FAIR

counts
5
10
4
10
3
10
2
10
10
LAND standard
LAND - zero time res.
10 cm plastic bars - zero time res.
1
-2 -1.5 -1 -0.5 0 0.5 1
∆t
(ns)
Figure 4.7.: Effect of passive detector material on the time resolution from a GEANT3
simulation of 470 MeV neutrons for three different detector configurations.
For ∆t the time of first detected light was subtracted from the time of the
first interaction.
4.2. From a Passive Converter to a Fully-Active Scintillator
Concept
The reason for the use of dense, passive converters, as technically realized for LAND
[Bla-92] by iron layers, has been, to improve the neutron efficiency via the detection
of particles created in the converter material. The passive material has an enhanced
interaction probability due to its higher density in comparison to scintillators. The active
scintillator layers combined with the passive converters serve to detect the interaction
products, mostly protons and γ-rays. In the following, we detail the difference in response
for a fully active detector and for a converter-based detector (LAND, 5 mm iron converter
layers) with respect to the achievable time resolution and to the efficiency. For a more
detailed description of LAND we refer to section 4.1.2 and [Bla-92].
A typical interaction of a high energy neutron with the iron converter leads to the production
of several protons, secondary neutrons and gammas. Since the protons mainly
come from evaporation of excited iron nuclei the angular distribution is isotropic and
typical proton energies are a few MeV with a high-energy tail. Therefore, the time resolution
is limited by the time jitter introduced by the protons traveling through the iron
until reaching the active part of the detector. The secondary neutrons from the primary
interaction have to pass part of the detector until they transfer energy to protons via
elastic scattering in the scintillator material. Again a time delay is introduced.
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