Physics¶

Note

For discussion of model and process naming, see issue 1562 .

The selection and data used to create physics classes will be defined underneath this struct.

struct Physics

Set up physics options.

Todo:

Move optical and hadronic physics options from celeritas::GeantPhysicsOptions

Move particle data from celeritas::ImportParticle

Add function for injecting user processes for celeritas::PhysicsParams

Move OpticalGenerator to OpticalGenPhysics or elsewhere

Todo:: How to better group these, especially when adding hadronic/photonuclear/decay/…?

Public Members

EmPhysics em: Physics that applies to offloaded EM particles.

MucfPhysics mucf: Muon-catalyzed fusion physics.

OpticalPhysics optical: Physics for optical photons.

Electromagnetic¶

struct EmPhysics

Electromagnetic physics processes and options.

Todo:: The ProcessBuilder is the “general” process builder type and should be refactored once import data is moved into the inp classes. The user_processes can be set externally or via FrameworkInput.geant.ignore_processes.

Public Members

BremsstrahlungProcess brems: Bremsstrahlung process.

PairProductionProcess pair_production: Electron+positron pair production process.

PhotoelectricProcess photoelectric: Photoelectric effect.

AtomicRelaxation atomic_relaxation: Atomic relaxation.

ProcessBuilderMap user_processes: Add custom user processes.

Optical¶

Optical photon generation is a part of the standard stepping loop that manages EM, decay, and hadronic physics, but its transport has its own separate stepping loop, where surface physics is the most complex part. Therefore, the OpticalPhysics input includes optical photon generation processes (such as Cherenkov and scintillation) and surface physics information. The latter describing how optical photons should interact with it.

struct OpticalPhysics

Optical physics processes, options, and surface definitions.

If scintillation or Cherenkov is enabled, optical photons will be generated.

Todo:: Move cherenkov/scintillation to a OpticalGenPhysics class.

Optical photon generation from EM particles

Todo:: Replace with physics input data

bool cherenkov = {false}: Generate Cherenkov photons.

bool scintillation = {false}: Generate scintillation photons.

Public Functions

inline explicit operator bool() const: Whether optical physics is enabled.

Bulk physics¶

struct OpticalBulkPhysics

Optical physics for bulk materials.

Each optical material can have one or more model that applies to it.

Public Functions

inline explicit operator bool() const: Whether optical physics is enabled.

Public Members

OpticalBulkModel<OpticalBulkAbsorption, IMC::absorption> absorption: Absorption models for all materials.

OpticalBulkModel<ImportMie, IMC::mie> mie: Mie scattering models for materials.

OpticalBulkModel<ImportOpticalRayleigh, IMC::rayleigh> rayleigh: Rayleigh scattering models for all materials.

OpticalBulkModel<ImportWavelengthShift, IMC::wls> wls: WLS models (TODO: merge into a single WLS with arbitrary components)

Surface physics¶

Celeritas’ surface physics implementation is designed differently from Geant4 and is meant to reduce code complexity and improve extensibility. In Geant4, a single model (e.g. “unified”) is a hard-coded combination of:

surface roughness types (e.g., polished or Gaussian),
reflectivity calculation (whether to use a grid or not), and
interaction description (e.g. dielectric-dielectric).

Celeritas separates these out into fully configurable surface physics inputs, so that a given surface ID must be present in three separate surface models. This “model unfolding” leads to a less simple input definition compared to Geant4, but it allows for user-extensible surface physics models. See Optical properties for further details of the importing.

struct OpticalSurfacePhysics

Surface physics definition.

Maps all optical surfaces with interaction models and surface parameters.

Interstitial materials are the interstitial materials per geometric surface. The last entry is used as the default surface.

Public Members

RoughnessModels roughness: Microfacet distribution models.

ReflectivityModels reflectivity: Reflectivity models.

InteractionModels interaction: Reflection+refraction+absorption models.

struct GridReflection

Model reflectivity as a user-prescribed function of energy.

As per Geant4 conventions:

Reflectivity is the probability that a photon undergoes its usual surface interaction.
Transmittance is the probability that a photon crosses the surface without change.
Efficiency is the quantum efficiency of a detector at the surface. It is the probability that a photon absorbed will be registered by the detector.

Only reflectivity and transmittance are defined by the user, while the remaining probability is the chance for the photon to be absorbed. If the photon is absorbed and an efficiency grid is defined, then the efficiency probability is used to determine if the photon is detected.

The reflectivity and transmittance grids are sampled together and must sum to between 0 and 1 at every grid point.

The efficiency grid is sampled independently (if provided), and must be in the range [0,1].

Public Members

Grid reflectivity: Grid values [MeV -> unitless].

Grid efficiency: optional

struct SmearRoughness

Global surface normal with smearing.

Roughness range is \( [0, 1] \), where 0 is specular and 1 is diffuse. This parameter is the complement of the polish as defined in Geant4:

roughness = 1 - GetPolish(); 

See celeritas::optical::SmearRoughnessSampler .

Note

This model is used by the GLISUR subroutine in Geant3 and the corresponding “glisur” surface model in Geant4.

Public Members

double roughness = {-1}: Scale from 0 = specular to 1 = diffuse.

struct GaussianRoughness

Approximate the microfacet normal distributions as Gaussian.

See celeritas::optical::GaussianRoughnessSampler .

Note

This model is used by the “unified” surface model in Geant4.

Public Members

double sigma_alpha = {0}: Standard deviation of the microfacet slope distribution.

struct ReflectionForm

Parameterization of the UNIFIED reflection model.

The reflection grids store the probability of each angular distribution in the UNIFIED model:

specular_spike : Reflection probability at the average surface normal.
specular_lobe : Reflection probability at the micro facet normal.
backscatter : Probability of backscattering after reflecting within a deep groove.

The sum of all three parameters must be <= 1 at every grid point, with the remainder being the probability of diffuse scattering.

Todo:: We could require these to all be on the same energy grid for improved performance and error checking.

Public Members

ReflectionGrids reflection_grids: Probability of reflection for each reflection mode.

Public Static Functions

static inline ReflectionForm from_spike(): Return a specular spike reflection form.

static inline ReflectionForm from_lobe(): Return a specular lobe reflection form.

static inline ReflectionForm from_lambertian(): Return a Lambertian (diffuse) reflection form.

static inline ReflectionForm from_mode(Mode only_mode): Construct a reflection form with only one active grid.

struct DielectricInteraction

Analytic interactions between dielectric and dielectric or metal materials.

Public Members

ReflectionForm reflection: Exiting angular distributions.

bool is_metal = {false}: Whether the interface is dielectric-dielectric or dielectric-metal.

Public Static Functions

static inline DielectricInteraction from_dielectric(ReflectionForm r): Return a dielectric-dielectric interaction.

static inline DielectricInteraction from_metal(ReflectionForm r): Return a dielectric-metal interaction.

Processes¶

struct BremsstrahlungProcess

Construct a physics process for bremsstrahlung.

Public Functions

inline explicit operator bool() const

TODO: macroscopic xs tables.

Whether process has data and is to be used

Public Members

SeltzerBergerModel sb: Lower-energy electron/positron.

RelBremsModel rel: High-energy electron/positron.

MuBremsModel mu: Muon (-/+)

struct PairProductionProcess

Construct a physics process for electron/positron pair production.

Public Functions

inline explicit operator bool() const: Whether process has data and is to be used.

Public Members

BetheHeitlerProductionModel bethe_heitler: Pair production from gammas.

MuPairProductionModel mu: Pair production from muons.

struct PhotoelectricProcess

Construct a physics process for photoelectric effect.

Public Functions

inline explicit operator bool() const: Whether process has data and is to be used.

struct AtomicRelaxation

Emit fluorescence photons/auger electrons from atomic de-excitation.

Todo:: Since multiple processes can cause the loss of a bound electron, we should have a separate “deexcitation” process that manages this efficiently. (Or perhaps a “generator” class to emit many simultaneously.)

Public Functions

inline explicit operator bool() const: True if data is assigned.

Public Members

std::map<AtomicNumber, ImportAtomicRelaxation> atomic_xs: Differential cross sections [(log MeV, unitless) -> millibarn].

Models¶

struct SeltzerBergerModel

Seltzer-Berger bremsstrahlung model.

Public Functions

inline explicit operator bool() const

TODO: microscopic elemental xs tables.

Whether model has data and is to be used

Public Members

std::map<AtomicNumber, inp::TwodGrid> atomic_xs: Differential cross sections [(log MeV, unitless) -> millibarn].

struct RelBremsModel

Relativistic bremsstrahlung model.

Public Functions

inline explicit operator bool() const: Whether model has data and is to be used.

Public Members

bool enable_lpm = {true}: Account for LPM effect at very high energies.

struct MuBremsModel

Muon bremsstrahlung model.

Public Functions

inline explicit operator bool() const: Whether model has data and is to be used.

struct BetheHeitlerProductionModel

Bethe-Heitler relativistic pair production from gammas.

Public Functions

inline explicit operator bool() const: Whether model has data and is to be used.

struct MuPairProductionModel

Pair production from muons.

Public Functions

inline explicit operator bool() const: True if data is assigned.

Public Members

MuPairProductionEnergyTransferTable muppet_table: Grid for sampling the energy of the electron-positron.

struct LivermorePhotoModel

Livermore photoelectric effect model.

Public Functions

inline explicit operator bool() const: Whether model has data and is to be used.

Public Members

std::map<AtomicNumber, LivermoreXs> atomic_xs: Tabulated microsopic cross sections [MeV -> b].

Grids¶

Tabulated physics data such as cross sections or energy loss are stored on increasing, sorted 1D or 2D grids. Both linear and spline interpolation are supported.

struct Grid

A grid of increasing, sorted 1D data.

This is used to store tabulated physics data such as cross sections or energy loss.

Public Static Functions

static inline Grid from_constant(double y): Construct a grid with a constant value for all real numbers.

struct UniformGrid: A uniform grid of increasing, sorted 1D data.

struct TwodGrid

An increasing, sorted 2D grid with node-centered data.

Data is interpolated linearly and indexed as ‘[x][y]’.

struct Interpolation

Interpolation options for the physics grids.

order is only used for poly_spline interpolation and bc is only used for cubic_spline interpolation.

Todo:: Let’s just use “Geant4 spline” and linear interpolation, and maybe it shouldn’t be inside every single grid?

Public Members

size_type order = {1}: Polynomial order for spline interpolation.

BC bc = {BC::geant}: Boundary conditions for calculating cubic spline second derivatives.