Physics III

Geant4 v9.4

Physics III

Makoto Asai (SLAC)Geant4 Tutorial Course

Contents• Cuts in Geant4 (a.k.a. production threshold)• Cuts per region• Decay process• Optical photon

Physics III - M.Asai (SLAC) 2

Geant4 v9.4

Cuts in Geant4(a.k.a. production threshold)

Secondary production threshold

• Every simulation user has to specify how low in energy his/her simulation should go

down.

– At what energy I should stop tracking a particle.

• This is a balancing act between the accuracy and the computing speed.

– 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, rays production

• In the traditional Monte Carlo code such as MCNP, GEANT3, EGS, Fluka, user

specifies energy cut-off.

– Track is killed when it comes down to this cut-off energy.

– Remaining energy is dumped at that point.


Secondary production threshold - continued

• But, such an absolute energy cut-off may cause imprecise stopping location

where energy is dumped.

• There is an issue of particle dependency.

– The range of 10 keV in Si is a few cm, while range of 10 keV e- in Si is a

few microns.

• There also is a material dependency.

– Suppose you have a sampling calorimeter which is made of lead and plastic

scintillator.

– If you adjust the energy cut-off to be suitable for lead, it must be too high for

scintillator and the result would be inaccurate. On the other hand, if you adjust

it for scintillator, it is too low for lead and thus you would waste CPU time.


Production threshold

• In Geant4, there is no energy cut-off (a.k.a. tracking cut).

– Tracks are always tracked down to a zero range/kinetic energy unless they

are killed by other reasons such as decay or interaction.

• Only the production threshold exists in Geant4, and it is applied only for the

processes which have infrared divergence.

– Some EM processes have infrared divergences which lead to an immense

number of smaller and smaller energy secondary gammas/electrons (e.g. in .

Bremsstrahlung, rays production).

• The user specifies the production threshold in range (length) rather than energy.

– Only the secondaries which can travel longer than this threshold are

generated.

– Lower energy secondaries are not generated and their energies are integrated

in the energy deposition of the parent track along its trajectory.Physics III - M.Asai (SLAC) 6

Production thresholds = cuts in range• Production threshold (i.e. range cut) represents the accuracy of location where

energy is deposited.

– Default value is 1 mm. You can set yours in your PhysicsList.

– Only one value of range cut is needed for each of e+, e- and gamma for all materials. Then the range cut is interpreted into energy threshold for each material.

• You may also define a cut for proton. It is used for all recoil ions by elastic scattering processes.

• Every track is tracked down to a zero kinetic energy. Thus, the stopping position is always correct.

– If a track no longer has enough energy to produce secondaries above the production threshold, following things happen.

• The discrete energy loss for this track ceases.

• The parent track still looses its kinetic energy through the continuous kinetic energy.


Geant4 v9.4

Cuts per region

Cuts per region

• Complex detector often has many different types of sub-detectors/structures.

– Finely segmented and very sensitive materials

– Large and undivided volumes such as support structure

• The uniform value of range cut for whole detector may not be appropriate

everywhere.

– For example, you may not need to spend some significant amount of time to

simulate an accurate EM shower inside a concrete shield.

• You may define regions (G4Region) of similar sensitivity/granularity in your

geometry.

– Regions should be defined in your detector construction.

– Then, assign range cuts to the regions.


Cuts per regionVoid PhysicsList::SetCuts()

{

SetCutValue(defaultCutValue, "gamma");

SetCutValue(defaultCutValue, "e-");

SetCutValue(defaultCutValue, "e+");

// Get the region

G4Region* aRegion = G4RegionStore::GetInstance()->GetRegion(“MyRegion");

// 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);

}


Geant4 v9.4

Decay process

Decay process

• Decay can happed in-flight or at-rest.

• G4Decay process is applicable to all unstable, long-lived particles.

– Decay of short-lived particles such as resonances are taken care within the

physics process which generates such particles. Thus, short-lived particle

won’t appear as a track.

• Decay process is different from other physics processes.

– For most processes, mean free path 1/ = N ρ σ / A

– For decay in-flight, = γ β c τ

• Single decay process for all eligible particles

– Retrieves branching ratio and decay models from decay table dedicated for

each particle type.


Decay channels

• Phase space

– 2-body e.g. 0 → , → p -

– 3-body e.g. K0L → 0 + -

• Many body

– Dalitz: 0 → ℓ+ ℓ-

• Muon decay and leptonic tau decay

– V−A theory, no radiative corrections, mono-energetic neutrinos

• Semi-leptonic K decay: K → ℓ


External Decayer

• Geant4 provides decay modes/channels for long-lived particles.

– You may modify decay channels if necessary.

• Decay modes for heavy flavor particles (c, b, t baryons, tau) are not provided by

Geant4.

– You may implement a decayer class extending the G4VExtDecayer abstract

class. Such an external decayer should be assigned to G4Decay.

– Examples/extended/eventgenerator/pythia/decayer6 example demonstrates

the use of Pythia-6 as an external decayer for heavy flavor.


Special decay processes

• G4DecayWithSpin

– For polarized muons

– Produces Michel positron spectrum with first-order radiative corrections

– Initial muon spin is required

– Propagates spin in magnetic field (precession) over remainder of muon

lifetime

• G4UnknownDecay

– Only for “unknown” particles (Higgs, SUSY, etc.)

– Discrete process: only in-flight decays allowed

– Pre-assigned decay channels must be defined


Pre-assigned decay products• Physics generator can assign a decay channel for each individual particle

separately, while in Geant4 you cannot specify a decay channel for each particle.

– Decay chain can be “pre-assigned”.

• A parent particle in the form of G4Track object travels in the detector, bringing “pre-assigned” decay daughters as objects of G4DynamicParticle.

– When the parent track comes to the decay point, pre-assigned daughters become to secondary tracks, instead of randomly selecting a decay channel defined to the particle type. Decay time of the parent can be pre-assigned as well.

Primary Particle - M.Asai (SLAC) 16

B-

G4Track

D0 -

K- +

pre-assigned decay products

K- +

D0

-

B-

K-

+

D0

Geant4 v9.4

Optical photon

Optical photon

• Technically, should belong to electromagnetic category, but

– Optical photon wavelength atomic spacing≫– Treated as waves: no smooth transition between optical and gamma particle

classes

– Represented by G4OpticalPhoton class which is independent to G4Gamma

• Optical photon production processes

– G4Cerenkov

– G4Scintillation

– G4TransitionRadiation

• WARNING: optical photons are generated without momentum/energy

conservation


Optical photon processes

• Photon can undergo

– Rayleigh scattering

– Refraction and reflection at medium boundaries

– Bulk absorption

– Wavelength shifting

• Polarization but not phase (no interference)

• Optical properties is to be specified in a G4MaterialPropertiesTable which is

assigned to G4Material

– reflectivity, transmission efficiency, dielectric constants, surface properties

• Spectral properties may be also specified in G4MaterialPropertiesTable

– scintillation yield, time structure (fast, slow components)


Optical boundary

• Reflection, refraction handled by G4OpBoundaryProcess

• Surface properties via G4OpticalSurfaceModel

– Boundary properties

• dielectric-dielectric

• dielectric-metal

• dielectric-black material

– Surface properties

• polished

• ground

• front- or back-painted, ...


Reflection and refraction

• Geant4 optical boundary process treats an optical photon as a particle.

– There is no “splitting”. The photon track undergo either reflection or refraction.


Wavelength shifting

• Taken care by G4OpWLS process

– Initial photon track is killed and secondary photon track with new wavelength

is created.

– G4OpWLS has its own physics table for mean free path

• You need to specify

– Absorption length as a function of photon energy

– Emission spectra parameters as a function of energy

– Time delay between absorption and re-emission


Absorption, scattering

• G4OpAbsorption

– Photon attenuation length from material properties

→ Mean free path

– Photon track is killed after a selected path length

• G4OpRayleigh

– Elastic scattering

– Polarization of initial and final photons

– Builds own physics table (for mean free path) using G4MaterialTable

– May only be used for optical photons

• See documentation in App. Dev. Guide: Optical Photon Processes


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