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5.1 Monte Carlo studies for the upgrade design 59
the incident ionizing particle. Therefore, the scintillation properties of
a material must be defined. These properties are the spectral distribution
of a possible slow and fast component of the scintillation light, the
refractive index and the absorption length as a function of the photon
energy. The scintillation light yield, fast and slow time constants and
the ratio of these time constants must also be defined. The values of
the wavelength dependent material properties are linked to the corresponding
momentum of the optical photon. The emission spectrum is
between 1.2399 eV (1000 nm) and 4.1329 eV (300 nm) [36]. The index of
refraction increases over this range of energy from 1.760 to 1.979 [50]. A
quenching factor of kB = 2.3·10 −3 gMeV −1 cm −2 , as quoted by Tretyak
[31] for CsI:Tl, was used.
Corresponding to these definitions, a number of Gaussian-distributed
optical photons are initialized isotropically at the point of interaction.
These photons can be absorbed or reach the crystal surface. At medium
boundaries, optical photons can be reflected or refracted. For a perfectly
smooth surface between two dielectric mediums, the refractive
index is sufficient. All other surfaces must be defined separately. The
Geant4 surface can be of two types, a skin surface (surrounding one
logical volume) and a border surface (being the boundary between
two logical volumes). An optical surface requires the definition of a
G4OpticalSurfaceModel, a G4OpticalSurfaceFinish (polished, ground,
polished- and ground-, back- or front-painted) and to specify the surface
as boundary between dielectric-metal, or dielectric-dielectric materials.
The reflectivity of the surface can be defined between 0.0 (no
reflection) and 1.0 (no transmission). In this simulation, a 0.5 mm layer
of MgO surrounds the crystal. This boundary surface was defined as
polished-frontpainted (polished surface with reflector attached without
air gap) with a reflectivity of 0.98 for MgO [28]. The optical photons can
reach the readout device directly or after several reflections at the crystals
boundary.
The Al layer, presenting the photo cathode, is defined as sensitive detector
volume. The detection of optical photons differs from that of
other particle classes. The optical photons are not detected after absorption,
but in the optical boundary process by passing the boundary
surface. The detection is based on the definition of the detection efficiency
on the skin of the sensitive volume. The boundary between the
glass and the Al layer is defined as polished and dielectric-metal. The
reflectivity is zero and the absorption efficiency is defined as shown in
figure 4.10.
The quantum efficiencies for seven readout devices are defined, which
are shown in figure 5.2. There are quantum efficiencies belonging to