a design study for a cobra upgrade to - Institut für Kern- und ...

62 5 Scintillator upgrade
vice. The length of the light guide depends on the number of devices
and the readout size. A visualisation, using the visualisation system
Heprep, is shown in fig. 5.3.
With this developed Geant4 application, different aspects of the scintillation
detector preparation were studied.
DATA ANALYSIS
Before starting the comparison between the readout devices, the influence
of the housing finish, reflectivity and absorption length was studied.
Therefore all 6 different kinds of housing finish were simulated
with a PMT readout, covering the front faces of the crystal. The finish
”polished” yields highest amount of photo electrons. Ground, groundand
polished-frontpainted yield around 75 % of the previous amount
of photo electrons. The ground- and polished-backpainted finish only
around 60 % of the photo electron yield, obtained with a polished surface.
The influence of the surface finish on the uniformity of the signal for
different source positions was not studied, but a higher uniformity of
light collection for ground, than for polished surfaces is expected [29].
The further simulations were carried out with a ground surface finish
and a reflectivity of 0.98.
As main examination, the amount of photo electrons detected by different
readout devices was evaluated. Therefore these 7 readout devices
were simulated with different sizes of active area. In a first simulation,
the readout devices covered the entire front facing crystal side, in
another simulation, only commercially available sizes for the readout
devices were taken into account. 10000 events with 662 keV photons
were simulated and the amount of produced photo electrons was only
recorded if the incident photon was completely absorbed. This was
the case for about 5900 events. The mean values of the recorded photo
electron number and the relative standard deviation for the different
readout areas and quantum efficiencies are listed in table 5.5.
RESULTS
The crucial point of this simulation is the limited knowledge about the
actual optical properties of individual crystals. This simulation can
therefore give only an estimation of the expected numbers of detected
photo electrons. Properties, like the optical absorption length, depend
on the crystal quality and production process. The increase of the optical
absorption length from 10 cm to 100 cm, improves the simulated
amount of detected photons significantly by almost a factor of 10. This
simulation is nevertheless useful for comparing the spectral matching
and the influence of the size of different readout devices.