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