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

5.3 Monte Carlo studies of the designed upgrade 67
tative.
5.3.1 Implementation of CsI detectors in Venom
VENOM is the Cobra simulation framework, first developed by H. Kiel
[8] and redesigned by J. Wilson and B. Morgan [53]. It is a Geant4 application,
enabling reproducible simulations of signal and background
physics processes in the detector for all COBRA collaborators. Venom
contains packages, which can be added and extended by the user. All
packages use the same physics class (VXPhysicsList) and material
class (VXMaterials). Geometry, action, readout and generator classes
can be added by writing new packages.
THE SCINT64 PACKAGE was written as part of this diploma thesis. It
is a new Venom package, constructing a simplified scintillator upgrade
design. The simulated construction does not consider the cable and
Teflon tubes, neglects the delrin layers, that hold the upper CsI crystals
and simplifies the read out electronics of the scintillators. In the Geant4
simulation, twelve scintillator blocks of 5×5×20 cm 3 and eight cubes
of 5×5×5 cm 3 surround a delrin Nest. These are attached to one (for
cubes) or two (blocks) borosilicate glass, representing the PMT window.
The other four or five sides of the scintillator are painted with a 0.5 mm
thick MgO layer. Only a preliminary Nest-design, instead of the current
design is implemented. It consists of 2 mm delrin plates and has an
open front side. A visualisation of the constructed assembly is shown
in figure 5.5.
The Scint64 package uses the PhysicsList of Venom. It considers therewith
a wide range of physical processes [53], without considering optical
photons. The simulations of the previous section showed, that
simulations of a few 10000 optical photons per deposited MeV particle
energy, do enhance the simulation time significantly. As the background
estimations for rare processes can require the simulation of several
hundred million of events, a description without optical photons
was preferred. The energy deposition in each crystal is recorded separately.
This data is called raw Venom data and provides no energy
resolution.
In this package, the quenching of the scintillation light output
for alpha particles is also considered and used a Birks factor of
kB = 2.3 · 10 −3 gMeV −1 cm −2 for CsI:Tl as reported by Tretyak [31].
At run time, a reduced energy of the alpha particles, according to their