NeuLAND - FAIR

spectrum, absorption length, refraction index, decay times, scintillation yield etc.. In
order to determine reflection properties, we made use of the possibility of defining optical
borders in GEANT4. In this simulation a polished dielectric-metal border between the
scintillator and a surrounding aluminum foil was chosen. Figure 4.10 shows the used
geometries for the light guides and some snapshots of photon tracking. The first row
shows the standard case with the PM directly coupled to the scintillator without a
light guide. The other rows show light guides with pyramidal, spherical, and parabolic
shapes. In the figure only the coupling to the 1 inch photomultiplier is shown but similar
simulations were performed for the 1.5 inch photomultiplier. For the scintillator module,
consisting of RP408 material, we chose the dimensions 5 × 5 × 200 cm. The light-output
was simulated based on a 50 MeV proton crossing the center of the scintillator bar. On
average 315000 optical photons were produced and in the table we list the numbers of
detected photons for the corresponding geometry for the photomultiplier with 1 and
1.5 inch, accordingly. The second far end of the scintillator was covered with a fictitious
photomultiplier covering the full area. On average 53000 photons were detected on this
”ideal read out” at this side.
We do not observe a substantial increase in read-out photons when applying light guides.
The photon tracking pictures displayed in the right column of figure 4.10 can serve to
interpret this effect as detailed in the following. Photons which are often reflected and
have large angles relative to the scintillator surface are often reflected back into the
scintillator bar by the light guide, while, in absence of a light guide, they had hit the
photomultiplier window. On the other hand, photons which travel almost parallel to
the scintillator but do not hit the photomultiplier directly have a high probability to
be reflected by the light guide into the direction of the photomultiplier. In total, the
number of photons hitting the photomultiplier stays almost constant. However, the
arrival time distribution of the photons is affected. This can be seen in Figure 4.11
where the simulated arrival time for the different light guide geometries is presented.
Relative to the case without light guide the amount of photons with short travel times
(”early” photons) are enhanced in cases with introduced light guides. Thus, the implementation
of light guides could lead to an improved time resolution since the fastest
photons determine the timing properties. For the 1 inch case, the light guides formed
like a truncated pyramid and the parabolic one, show very good performance. Following
closely these findings, we propose for the NeuLAND submodules light-guides of conical
shape for practical reasons, see section 5.2.2.
In order to obtain information on the number of photons expected close to the detection
threshold, electrons with an energy of 1 MeV were also simulated. In this case about
300 photons were detected with a 1 inch and 600 with a 1.5 inch photomultiplier.
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