Overview

This section summarizes key usage patterns and implementation details from Celeritas, especially those that differ from Geant4.

Units

The unit system in Celeritas is CGS with centimeter (cm), gram (g), second (s), gauss (Ga?), and kelvin (K) all having a value of unity. With these definitions, densities can be defined in natural units of \(\mathrm{g}/\mathrm{cm}^3\), and macroscopic cross sections are in units of \(\mathrm{cm}^{-1}\). See the units documentation for more descriptions of the core unit system and the exactly defined values for SI units such as tesla.

Celeritas defines constants from a few different sources. Mathematical constants are defined as truncated floating point values. Some physical constants such as the speed of light, Planck’s constant, and the electron charge, have exact numerical values as specified by the SI unit system [BureauIdPeMesures19]. Other physical constants such as the atomic mass unit and electron radius are derived from experimental measurements in CODATA 2018. Because the reported constants are derived from regression fits to experimental data points, some exactly defined physical relationships (such as the fine structure \(\alpha = \frac{e^2}{2 \epsilon_0 h c}\) are only approximate.

Unlike Geant4 and the CLHEP unit systems, Celeritas avoids using “natural” units in its definitions. Although a natural unit system makes some expressions easier to evaluate, it can lead to errors in the definition of derivative constants and is fundamentally in conflict with consistent unit systems such as SI. To enable special unit systems in harmony with the native Celeritas unit system, the Quantity class stores quantities in another unit system with a compile-time constant that allows their conversion back to native units. This allows, for example, particles to represent their energy as MeV and charge as fractions of e but work seamlessly with a field definition in native (macro-scale quantity) units.

Physics

Celeritas implements physics processes and models for transporting electron, positron, and gamma particles as shown in the accompanying table. Implementation details of these models and their corresponding Geant4 classes are documented in Physics distributions.

Electromagnetic physics processes and models available in Celeritas.

Particle	Processes	Models	Celeritas Implementation	Applicability
\(e^-\)	Ionisation	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
\(e^+\)	Ionisation	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
	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

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.

Geometry

Celeritas has two choices of geometry implementation. VecGeom is a CUDA-compatible library for navigation on Geant4 detector geometries. ORANGE is a work in progress for surface-based geometry navigation that is “platform portable”, i.e. able to run on GPUs from multiple vendors.

Celeritas wraps both geometry packages with a uniform interface for changing and querying the geometry state.

Stepping loop

The stepping loop in Celeritas is a sorted loop over “actions”, each of which is usually a kernel launch (or an inner loop over tracks if running on CPU).

GPU usage

Celeritas automatically copies data to device when constructing objects as long as the GPU is enabled.