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GEANT 4 Simulations

POLAR. GEANT 4 Simulations. Estela Suarez – Wojtek Hajdas. 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

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GEANT 4 Simulations

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  1. POLAR GEANT 4 Simulations Estela Suarez – Wojtek Hajdas PSI, 22 May 2007

  2. 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

  3. 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

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. Radiation Belt at GEO • We consider electrons. • Their energy distribution is according to the spectrum in the left

  10. 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.

  11. 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

  12. 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

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