EM Physics

The physics models in Celeritas are primarily derived from references cited by Geant4, including the Geant4 physics reference manual. Undocumented adjustments to those models in Geant4 may also be implemented.

• Continuous slowing down
  • calc_mean_energy_loss()
  • Energy loss fluctuations
• Ionization
  • Models
  • Distributions
• Bremsstrahlung
  • Models
  • Cross sections
  • Distributions
• Coulomb scattering
  • Models
  • Cross sections
  • Distributions
• Photon interactions
  • Scattering
  • Photoelectric effect
• Pair production
  • Models
  • Distributions
• Annihilation
  • Models
  • Cross sections

The implemented physics models are meant to match the defaults constructed in G4EmStandardPhysics. Known differences are:

• Particles other than electrons, positrons, and gammas are not currently supported.

• As with the AdePT project, Celeritas currently extends the range of Urban MSC to higher energies rather than implementing the Wentzel-VI and discrete Coulomb scattering.

• Celeritas imports tracking cutoffs and other parameters from G4EmParameters, but some custom model cutoffs are not accessible to Celeritas.

As extension to the various random distributions, Celeritas expresses many physics operations as distributions of updated track states based on original track states. For example, the Tsai-Urban distribution used for sampling exiting angles of bremsstrahlung and pair production has parameters of incident particle energy and mass, and it samples the exiting polar angle cosine.

All discrete interactions (in Geant4 parlance, “post-step do-it”s) use distributions to sample an Interaction based on incident particle properties. The sampled result contains the updated particle direction and energy, as well as properties of any secondary particles produced. Interaction limits are described in Step limits.

The following table summarizes the EM processes and models in Celeritas.

Electromagnetic physics processes and models available in Celeritas.

Particle	Processes	Models	Celeritas Implementation	Applicability
\(e^-\)	Ionization	Møller	celeritas::MollerBhabhaInteractor	0–100 TeV
	Bremsstrahlung	Seltzer–Berger	celeritas::SeltzerBergerInteractor	0–1 GeV
		Relativistic	celeritas::RelativisticBremInteractor	1 GeV – 100 TeV
	Coulomb scattering	Urban	celeritas::UrbanMscScatter	100 eV – 100 TeV
		Coulomb	celeritas::CoulombScatteringInteractor	0–100 TeV
\(e^+\)	Ionization	Bhabha	celeritas::MollerBhabhaInteractor	0–100 TeV
	Bremsstrahlung	Seltzer-Berger	celeritas::SeltzerBergerInteractor	0–1 GeV
		Relativistic	celeritas::RelativisticBremInteractor	1 GeV – 100 TeV
	Coulomb scattering	Urban	celeritas::UrbanMscScatter	100 eV – 100 TeV
		Coulomb	celeritas::CoulombScatteringInteractor	0–100 TeV
	Annihilation	\(e^+,e^- \to 2\gamma\)	celeritas::EPlusGGInteractor	0–100 TeV
\(\gamma\)	Photoelectric	Livermore	celeritas::LivermorePEInteractor	0–100 TeV
	Compton scattering	Klein–Nishina	celeritas::KleinNishinaInteractor	0–100 TeV
	Pair production	Bethe–Heitler	celeritas::BetheHeitlerInteractor	0–100 TeV
	Rayleigh scattering	Livermore	celeritas::RayleighInteractor	0–100 TeV
\(\mu^-\)	Ionization	ICRU73QO	celeritas::MuHadIonizationInteractor	0–200 keV
		Bethe–Bloch	celeritas::MuHadIonizationInteractor	200 keV–1 GeV
		Mu Bethe–Bloch	celeritas::MuHadIonizationInteractor	200 keV–100 TeV
	Bremsstrahlung	Mu bremsstrahlung	celeritas::MuBremsstrahlungInteractor	0–100 TeV
	Pair production	Mu pair production	celeritas::MuPairProductionInteractor	0.85 GeV–100 TeV
\(\mu^+\)	Ionization	Bragg	celeritas::MuHadIonizationInteractor	0–200 keV
		Bethe–Bloch	celeritas::MuHadIonizationInteractor	200 keV–1 GeV
		Mu Bethe–Bloch	celeritas::MuHadIonizationInteractor	200 keV–100 TeV
	Bremsstrahlung	Mu bremsstrahlung	celeritas::MuBremsstrahlungInteractor	0–100 TeV
	Pair production	Mu pair production	celeritas::MuPairProductionInteractor	0.85 GeV–100 TeV

Imported data

In addition to the core Imported data, these import parameters are used to provide cross sections, setup options, and other data to the EM physics.

struct ImportEmParameters

Common electromagnetic physics parameters (see G4EmParameters.hh).

Note

Geant4 v11 removed the Spline() option from G4EmParameters.hh.

Note

The Geant4 MSC models use the values in G4EmParameters as the defaults; however, the MSC parameters can also be set directly using the model setter methods (there is no way to retrieve the values from the model in that case).

struct ImportAtomicTransition

EADL transition data for atomic relaxation for a single element.

struct ImportAtomicSubshell

struct ImportAtomicRelaxation

struct ImportLivermoreSubshell

Livermore EPICS2014 photoelectric cross section data for a single element.

struct ImportLivermorePE

struct ImportMuPairProductionTable

Sampling table for electron-positron pair production by muons.

This 3-dimensional table is used to sample the energy transfer to the electron-positron pair, \( \epsilon_p \). The outer grid stores the atomic number using 5 equally spaced points in \( \log Z \); the x grid stores the logarithm of the incident muon energy \( T \) using equal spacing in \( \log T \); the y grid stores the ratio \( \log \epsilon_p / T \). The values are the unnormalized CDF.

using celeritas::ImportSBTable = inp::TwodGrid

Seltzer Berger differential cross sections for a single element.

This 2-dimensional table stores the scaled bremsstrahlung differential cross section [mb]. The x grid is the log energy of the incident particle [MeV], and the y grid is the ratio of the gamma energy to the incident energy.

DEPRECATED: remove in v1.0.