Development of an Interface for Using EGS4 Physics Processes in Geant4

1
Development of an Interface for Using EGS4
Physics Processes in Geant4

• K.Murakami (KEK)
• Koichi.Murakami_at_kek.jp
• 27/Mar./2003
• 2003 Conference for Computing in High Energy and
  Nuclear Physics (CHEP03) _at_UCSD

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Outline

1. Introduction
2. System Analysis
3. Implementation
4. Status and Summary

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I. Introduction
4
Introduction

• Geant4 has rich flexibility and expansibility for
  adding physics processes, in addition to its
  powerful geometry description.
• It is not easy to describe complicated/realistic
  geometry for EGS4 users.
• Taking advantage of the capabilities of Geant4 as
  a framework, we developed an interface for using
  EGS4 as a module of physics process in Geant4.

• By means of this interface,
• EGS4 users can share Geant4 powerful resources,
  such as geometry description, tracking etc.
• A common environment for comparison tests between
  EGS4 and Geant4 especially in users realistic
  application level.

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Co-Workers

• Persons involved
• K.Murakami (KEK, Koichi.Murakami_at_kek.jp)
• K.Amako (KEK)
• H.Hirayama (KEK)
• Y.Namito (KEK)
• T.Sasaki (KEK)
• M.Asai (SLAC)
• T.Koi (SLAC)

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II. System Analysis
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Analysis of EGS4 System- in terms of difference
from Geant4-

• We first carried out analysis of the EGS4 system
  in terms of difference from Geant4.
• Our policy is that we should care for NOT to
  change algorithms and prescriptions which are
  used in the EGS4 system.
• Issue-1 A way of calculation of step size and
  process selection
• In Geant4, each process has its own fate in terms
  of NIL (N of Interaction Length). Then, a process
  having minimum fate will occur.
• In EGS4, the next step size is calculated based
  on the total cross section of a particle. Then,
  which process will occur is chosen at the rate of
  branching fraction of a process.
• In principle, these work in the same manner.
  Good!!
• We implement the EGS physics processes as single
  Geant4 processes for electron/photon
  respectively.

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Analysis of EGS4 System (Contd)

• Issue-2 Scheme of describing material
  information
• In Geant4,
• Material information has only properties of
  material itself (atomic number/mass, density,
  composition...)
• Cross section tables are calculated at
  initialization time.
• In EGS4,
• Users have to prepare a material (cross section)
  file by executing PEGS, which is an external
  program, preceding to execution of a EGS4
  program.
• Additional information related to processes
  (cutoff, parameters/coefficients of formulas) are
  described as material.
• We manage material information in terms of both
  of Geant4 and EGS4.
• Users have to execute PEGS externally and our
  interface manages material information listed in
  a PEGS output file.
• A mapping table between G4 materials and EGS4
  materials is prepared.
• Issue-3 Cutoff scheme
• In Geant4,
• There is no cutoff energy. Particles basically
  will be transported until they have zero kinetic
  energy.
• An idea of production cut is introduced to avoid
  infrared divergence.
• In EGS4,
• An idea of cutoff energy is applied. Particles
  below cutoff energy will be discarded.

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Table of Components Which package is used

• Control Framework EGS4 Geant4
• Tracking EGS4 Geant4
• Geometry EGS4 Geant4
• Materials EGS4 Geant4
• Cross Section Table EGS4 Geant4
• (PEGS)
• Physics Process EGS4 Geant4
• Cutoff Scheme EGS4 Geant4

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III. Implementation
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Functional Mapping

• EGS4 Flow
• SHOWER ?
• (tracking controller)
• ELECTOR ?
• PHOTON
• (controller for stepping and
• process invocation)
• HOWFAR ?
• (geometry and navigation,
• user code)
• each Physics Process ?
• COMPT
• BREMS
• ...

• Geant4 Class
• tracking manager
• stepping manager
• single process for electron
• single process for photon
• GPIL() (GetPhysicalInteractionLength)
• calculating the next interaction point
• DoIt()
• describing a reaction
• G4 geometry
• G4 transportation (as a process)
• reused as-is
• wrapped with FORTRAN-C interface

EGS4 subroutines
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How It Works
g4change
g4mate
egsmate

• A virtual volume with
• no dimensions
• empty media

evacuation
update
g4-egs material map
egs stack
g4track
egs stack
egs stack
update

• In the EGS4 world, a single virtual volume with
• no dimensions (transportation is a G4 job)
• empty medium (dynamically updated)
• is prepared.
• At tracking time,
• Medium information of the EGS4 volume is updated
  at every step though the material map from G4 to
  EGS4.
• Kinematical information in the EGS4 stack is
  updated from G4 track
• A EGS4 process is invoked.
• The stack information of EGS4 is evacuated to a
  G4 particle-change.

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Material Description
G4-EGS Material Map
EGS4 World
PEGS Material
EGS Material
Geant4 World
G4Material

• There are three classes describing materials
• PEGS Material
• It describes materials defined in a PEGS output.
• EGS Material
• It has a PEGSMaterial.
• In addition, it has information, such as
• user cutoffs, user density, user flags, ...
• G4 Material
• A G4Material is related to a EGSMaterial by users
  at initialization time.
• A G4Material is translated to a EGSMaterial
  through the map at tracking time.

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Process List Currently Covered

• As the first target, we started with interfacing
  with a plain EGS4 (distributed as a KEK version).

• Photon processes
• Rayleigh scattering
• pair production
• photo-electric effect
• w/ subsequent fluorescence
• Compton scattering

• Electron processes
• multiple scattering
• momentum flip only.
• lateral displacement is not included.
• PRESTA is not included.
• continuous energy loss
• Bremsstrahlung
• Møller scattering (e-)
• BhaBha(e)
• positron annihilation (e)
• (in-flight / at-rest)

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View from Users Side

• User Physics List
• An alternative EM-physics list is provided as a
  modular Physics List for EM w/ EGS4 processes.
• User Detector Construction (Geometry)
• Users dont have to modify any existing Geant4
  geometry codes.
• Users make assignment of a Geant4 material to a
  EGS4 material by hands.
• Cross Section Table
• Users have to execute PEGS externally.
• define PEGS materials in PEGS input files.
• execute PEGS to generate a cross section file
  (so-called PEGS output).
• All operations can be performed via G4 command
  line interface interactively.
• assigning G4Material to EGSMaterial
• setting users parameters
• initialization, ...

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IV. Status and Summary
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Sample Event Display

• 5 electrons with Ek50 MeV into a slab geometry
  composed of several materials. (w/ cutoff Ek10
  keV)

red electron green photon
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Benchmarks

• Development environment
• Linux (SuSE / RedHat / ...)
• gcc/g77 ver. 3.2 (2.95.2/3)
• Geant4 ver.5.0 (patch01)
• EGS4 (distributed as a KEK version)
• Robustness
• several-M events in a simple geometry were
  successfully generated without crush.
• Detail benchmark tests were just started.

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Current Status and Plans

• First implementation was finished.
• Detail system check and benchmark tests are
  started as the next target.
• We have plans of comparisons
• between EGS4 itself and the EGS4-G4 interface.
• between G4 and EGS4(w/ the interface) on common
  geometries.
• Technical study for treating PRESTA is planned
  for more precise simulation.
• The treatment of multiple scattering is closely
  coupled with geometry (boundary).

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Summary

• We have successfully developed the first version
  of an interface between EGS4 and Geant4 based on
  OO-approach.
• This work is a proof of flexibility and
  expansibility of the Geant4 framework.
• By means of this interface, EGS4 users will be
  able to share Geant4 powerful resources, such as
  geometry description, tracking etc.
• Further detail benchmarks are on going.