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

5.1 Monte Carlo studies for the upgrade design 53
cost and the amount of needed shielding material, smaller scintillators
would be preferable. Whereas, a good detection efficiency for gamma
rays requires bigger scintillators. For finding an optimal size of NaI:Tl,
CsI:Tl, BGO and CWO crystals, the absorption efficiency of different
scintillator crystals for gamma rays was determined with the simulation
”ScintBlock”.
GEANT4 APPLICATIONS ”SCINTBLOCK”
The setup of this simulation consists of a volume of 2.5×2.5×5 m 3 filled
with air. Within this, a cage of 2×2×4 m 3 containing nitrogen gas and,
as default, a 1×1×1 m 3 CsI scintillator block is positioned. The user can
change the size and the material of the target interactively via the commands
/SB/det/setScintLength and /SB/det/setScintMat.
The kind of the simulated particles and the physic processes are set
in the PhysicsList class. All electromagnetic processes are defined for
photons, charged leptons and charged hadrons. The primary kinematic
consists of a single particle which hits the target perpendicular to the
input face. The type of the particle and its energy are set in the PrimaryGeneratorAction
class, and can be changed via the G4 build-in
commands /gun/particle and /gun/energy of the ParticleGun
class. As default a single 511 keV photon is initialised.
SIMULATION AND DATA ANALYSIS
Initially, a record of the number of the event (event ID), the number
of the particle in the respective event (track ID), the kind of the particle
(particle), the initial energy (Ei), the place of the interaction (x,y,z),
the energy deposited in the scintillator (Edep), the kind of the interaction
(process) and the amount of recorded steps (step) was printed for
every step with an interaction in the scintillator volume. In order to
understand the tracking of a particle, this detailed output was used to
reconstruct samples of simulated events.
An event of the simulation of a 500 keV gamma ray in NaI is shown
in table 5.1. The initial photon is back-scattered and produces three
Compton electrons (track ID 4, 3 and 2) before leaving the scintillator.
This is recorded as transportation process (trans). The Compton electrons
number 4 and 3 loose their energy by ionization processes (eIoni)
and the electron number 2 emits Bremsstrahlung (eBrems) before being
absorbed. The Bremsstrahlungs photon is absorbed by the photo effect
(phot) and the produced photo electron (track ID 6) is stopped nearby.
In total 840 simulations with different macros were started, changing
the photon energy, the crystal material and the crystal size. For each
simulation, the impact of 50000 monoenergetic photons (one after the