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.

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.

• Summary
• Continuous slowing down
  • calc_mean_energy_loss()
  • Energy loss fluctuations
  • Integral rejection
• 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

Imported data

Celeritas reads physics data from Geant4 (or from a ROOT file exported from data previously loaded into Geant4). Different versions of Geant4 (and Geant4 data) can be used seamlessly with any version of Celeritas, allowing differences to be isolated without respect to machine or model implementation.