GEANT 4 Simulations
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Transcript of GEANT 4 Simulations
GEANT 4 Simulations
Estela Suarez – Wojtek Hajdas
POLAR
PSI, 22 May 2007
Index
• General Description of the simulations• Uniformity studies• Scintillator roughness
– Simulation expectations– Experimental measurements
• New particle generator• Radiation Belts at GEO implementation
– Description of the spectrum– Study of the influence of shielding
General Description• GEANT4 simulations
• Physics Processes Included:– Gamma: Low Energy (LE) Polarized Compton, LE Gamma Conversion, LE Photoelectric.– Electron: Multiple Scattering, eIonisation, eBremmstrahlung.
• Inputs: Type of particle, number of particles lunched and their energy distribution.
• Outputs: Event-table for interacting photons: – Initial energy of the gamma– Energy deposited– Scintillator bar fired (its number)– Number of times the gamma interacted in the same bar
• Simulations for 3 different detectors:– POLAR Prototype → 8x8 scintillator bars (6x6x200 mm each one)– BIG Polar (whole detector) → 5x5x64 bars (25 PMT)– 1 Bar with different coatings → For light collection studies
Uniformity studies
• Lack of uniformity affects the modulation factor.
• 100% polarized photons coming from above to a central bar of POLAR.
• Conclusions:– Non-Uniformity of PM has
a strong influence. – Poor energy resolution
from scintillators does not influence so strongly.
100% Modulation factor vs. Uniformity
0
10
20
30
40
50
0 100 200 300
Energy (keV)
Mod
ulat
ion
fact
or (%
))
Non-UniformIdealEnergy Smearing
Scintillator Roughness
• The Light collection is strongly reduced when the index of roughness increases.
• On the other hand, the roughness of the wrapping material does not affect the result.
Scintillator Roughness: Measurements
• Measurement of the good face of scintillator.
• Standard deviation of height distribution 50 Å
• 5nm/423nm =0.012 < 0.025
• According to Levin & Moisan 1996 Crystal can be considered highly polished.
Scintillator Roughness: Measurements
• Measurement of the bad face of scintillator.
• Standard deviation of height distribution 150 Å
• 15nm/423nm = 0.035 0.025
• According to Levin & Moisan 1996 fairly good polishing.
Particle generator• Particles uniformly
distributed on a semi-sphere (R=50 cm).
• From each point the origin of particles is randomized on the surface of a circle (r=30cm).
• Kind of particles used, and their energy can be selected.
Radiation Belt at GEO
• We consider electrons.
• Their energy distribution is according to the spectrum in the left
Radiation Belt at GEO: shielding
• POLAR placed into an aluminum box which walls have a defined thickness.
• Calculated: number of electrons that are detected one or several times, taking in account the threshold:
5keV < Edep < 500keV.
Radiation Belt at GEO: Results
• No shielding: 200 keV deposited
per event on average
• 1 mm shielding– Only 20% of e- arrive
to the scintillators 64 keV deposited
per event on average
Radiation Belt at GEO: Results
• 106 events simulated• Electrons distributed
in 2.
• Aprox. 10000 coinc expected from a GRB
• This Background would kill the signal
Al-shieldThickness
(mm)Nc/sec
Nc/sec 1bar
Nacc/sec pair
Nacc/sec POLAR
0 4.2E+09 2612534 273013 546026608
1 8.6E+07 53516 115 229119
3 3.3E+07 20546 17 33770
5 3.0E+07 18726 14 28054