Geant4 Process C-S Biasing

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Geant4 Process C-S Biasing

• GEANT4 Workshop, Triumf
• Fan Lei
• Space Department, QinetiQ
• 03-09-2003

2
Contents

• Introduction
• Algorithm
• Design and Implementation
• How to use
• Examples
• Discussions

3
Introduction

• C-S biasing is a technique to artificially
  enhance/reduce the interaction cross-section of a
  particular process, in order to improve the
  statistics of a simulation (variance reduction) .
• Use case
• Thin layer interactions (enhancement)
• Thick layer shielding penetration (reduction)
• ...
• Other similar variance reduction techniques
  forced collisions path length stretching

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Algorithm

• At each step
• If the biased process is being selected, the
  weights of current track and the secondaries are
  changed to
• w (1 exp(-s/x))/(1-exp(-fs/x) w
• If the process is not being selected, the weight
  of the current track is changed to
• w exp(-(1-f)s/x) w
• where w the original parent particle weight
• s the step length
• x the unbiased current interaction length of
  the process
• f the enhancement factor

5
Design

• No direct impact to existing process
  implementations.
• Use the wrapper approach
• G4VWrapperProcess( ) already in G4 kernel
• G4XWrapperProcess( ) new
• Method to set enhancement factor
• Modify the returned interaction length
• Modify the particle weights in PostStepDoIt(),
  AtRestDoIt()
• Modify the parent particle weight in
  AlongStepDoIt()

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G4XWrapperProcess
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G4XWrapperProcess(cont.)
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How to use

• aProcess // the process to be biased
• pManager // the process manager
• G4XWrapperProcess fWProcess new
  G4XWrapperProcess()
• fWProcess -gt RegisterProcess(aProcess)
• fWProcess -gt SetEnhanceFactor(2.) // enhanced by
  a factor of 2.
• pManager -gt AddDiscreteProcess(fWProcess)
• pManager -gt SetProcessOrdering(fWProcess,
  idxAlongStep)
• pManager -gt SetProcessOrdering(fWProcess,
  idxPostStep)

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Application Example 1

• Set-up
• 0.1 mm Al target with 200 keV gamma rays.
• PHS in the target and gamma spectrum behind the
  target are recorded
• Simulations
• 107 gammas, unbiased
• 105 gammas, unbiased
• 105 gammas, c-s of photoelectric and Compton
  processes enhanced by a factor of 10.

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Application Example 1
Absorption and transmission spectra normalised
to per incident gamma
1
1
2
2
3
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Energy (kev)
Energy (kev)
Gamma
PHS
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Application Example 2

• Set-up
• 0.1 mm Al target with 2 MeV electron beam.
• PHS in target and gamma spectrum behind the
  target are recorded
• Simulations
• 106 electrons, unbiased
• 105 electrons, unbiased
• 105 electrons, c-s of bremsstrahlung process
  enhanced by a factor of 10.

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Application Example 2
Target PHS and emitting gamma spectrum
normalised to per incident e-
1
1
2
2
3
3
Energy (kev)
Energy (kev)
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Application Example 3

• Set-up
• 0.1 mm Al target with 20 keV neutrons.
• Absorption and transmission spectra are recorded
• Simulations
• 105 neutrons, unbiased
• 105 neutrons, elastic scattering c-s enhanced by
  a factor of 10
• 105 neutrons, elastic scattering c-s enhanced by
  a factor of 100

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Application Example 3
1
1
2
2
3
3
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Discussions

• Simple implementation based on G4VWrapperprocess
• No change to existing process required and easy
  to use
• Interaction length can be stretched instead
• Process based only most hadron physics are
  model based
• Multiple interactions in the volume of interest
• Optimal enhancement factor not easy to determine
  
• Forced interaction Interact once only in a
  volume
• Different design/implementation required
• Will be a boundary process