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Файл:Моделирование (1 сем, мага) / geant4 / Geant4_3
.pdf
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Summary
•All the particles, physics processes, and production cuts
needed for an application must go into a physics list
•Two kinds of physics list classes are available for users to
derive from
•G4VUserPhysicsList – for relatively simple physics lists
•G4VModularPhysicsList – for detailed physics lists
•Some pre-packaged physics lists are provided by Geant4
as starting points for users
•electromagnetic physics lists
•electromagnetic + hadronic physics lists
•Care is required by user in choosing the right physics to
use
Production thresholds
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Secondary production threshold
•Every simulation developer must answer the question:
how low in energy can you go?
•at what energy do I stop tracking particles ?
This is a balancing act
•need to go low enough to get the physics you're interested
in
•can't go too low because some processes have infrared
divergences causing CPU time to explode
•eg. Bremsstrahlung, delta rays production
•The traditional Monte Carlo solution is to impose an
absolute cutoff in energy (eg. Geant3)
•particles are stopped when this energy is reached
•remaining energy is dumped at that point
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Secondary production threshold
•But, such an absolute cut-off in energy may cause imprecise
stopping location and deposition of energy
•There is also a particle & interaction dependence
the range of 10 keV γ in Si is a few cm
different from the range of 10 keV e- in Si which is a few
microns
•And a material dependence, eg sampling calorimeter:
suppose you have a detector made of
alternating sheets of Pb and plastic scintillator
you set cut-off for Pb, it will likely be wrong for scintillator
which does the actual energy deposition measurement
you set cut-off for scintillator, Geant4 will spend excessive time
tracking shower particles in Pb
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Secondary production threshold
•In Geant4 there are no tracking cuts
• particles are tracked down to a zero range/kinetic energy
•Only production thresholds exist
• i.e. cuts allowing a particle to be created or not
•Why are such cuts needed ?
• Some electromagnetic processes involve infrared divergences
• this leads to a huge number of smaller and smaller energy
photons/electrons (such as in Bremsstrahlung, d-ray production)
• production cuts limit this production to particles above the
threshold only
• the remaining, divergent part, is treated as a continuous effect
(i.e. AlongStep action)
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Secondary production threshold
•Geant4 solution: impose a cut in range
default value is 1 mm, you can set yours in your PhysicsList the primary
particle loses energy by producing secondary electrons or gammas
if the primary no longer has enough energy to produce secondaries
which travel at least this cut in range (1 mm), two things happen:
• the discrete energy loss ceases: no more secondaries are produced
• the primary is tracked down to zero energy using continuous energy loss
•Stopping location is therefore correct
•Only one value of cut in range is needed for all materials because it
corresponds to different production thresholds in energy (depending on
material).
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Cuts per Region
•Complex detectors often have many types of subdetectors
Finely segmented volumes
Very sensitive (sampling or readout) materials
Large, undivided volumes
Inert materials
•The same value of cut in range may not be appropriate
everywhere. The user can:
•define regions (G4Region) of similar sensitivity and granularity
in his geometry (DetectorConstruction)
•assign a different cut in range for each region
directly using C++ in PhysicsList
or via UI commands
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Cuts per Region in PhysicsList
Void PhysicsList::SetCuts()
{
SetCutValue(defaultCutValue, "gamma");
SetCutValue(defaultCutValue, "e-");
SetCutValue(defaultCutValue, "e+");
// Get the region
G4Region* aRegion =G4RegionStore::GetInstance()>GetRegion("NewRegion");
// Define cuts object for the new region and set values
G4ProductionCuts* cuts = new G4ProductionCuts;
cuts->SetProductionCut(0.01*mm); // for gamma, e+, e-
// Assign cuts to region
aRegion->SetProductionCuts(cuts);
}
•Note : one may also find cut for protons which is used as cut for all
recoil ions by elastic scattering processes
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The decay process
•Decays can happen in-flight or at rest
•Should be applied to all unstable, long-lived
particles
•Different from other physical processes
For most processes, mean free path 1/λ = N ρ σ / A
For decay in-flight, λ = γ β c τ
•Same decay process for all eligible particles
Retrieves BR and decay modes from decay table
stored with particle type
Implemented as subclass of G4VRestDiscreteProcess
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Available decay channels
•Phase space
2-body
3-body
Many body
•Dalitz
•Muon decay and leptonic tau decay
• V−A theory, no radiative corrections, mono-
energetic neutrinos
•Semi-leptonic K decay
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