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Physics and simulation inputs

This guide collects the Geant4 optical-surface notes, Simphony source configuration, and the input requirements that were previously documented in the top-level README.

Optical surface models in Geant4

In Geant4, optical surface properties such as finish, model, and type are defined using enums in the G4OpticalSurface and G4SurfaceProperty header files:

These enums allow users to configure how optical photons interact with surfaces, controlling behaviors like reflection, transmission, and absorption based on physical models and surface qualities. The string values corresponding to these enums, such as ground, glisur, and dielectric_dielectric, can also be used directly in GDML files when defining <opticalsurface> elements for geometry. Geant4 parses these attributes and applies the corresponding surface behavior.

For a physics-motivated explanation of how Geant4 handles optical photon boundary interactions, refer to the Geant4 Application Developer Guide - Boundary Process.

<gdml>
  ...
  <solids>
    <opticalsurface finish="ground" model="glisur" name="medium_surface" type="dielectric_dielectric" value="1">
      <property name="EFFICIENCY" ref="EFFICIENCY_DEF"/>
      <property name="REFLECTIVITY" ref="REFLECTIVITY_DEF"/>
    </opticalsurface>
  </solids>
  ...
</gdml>

Torch configuration

GPUPhotonSource and GPUPhotonSourceMinimal read photon source parameters from a JSON config file, by default config/dev.json.

Field Description
type Source shape: disc, sphere, point
radius Size of the source area (mm)
pos Center position [x, y, z] (mm)
mom Emission direction [x, y, z] (normalized automatically)
numphoton Number of photons to generate
wavelength Photon wavelength (nm)

Code differences

Feature GPUCerenkov GPURaytrace GPUPhotonSource GPUPhotonSourceMinimal GPUPhotonFileSource
Cerenkov genstep collection Yes Yes No No No
Scintillation genstep collection No Yes No No No
Wavelength shifting (WLS) Yes Yes Yes Yes Yes
Torch photon generation No No Yes Yes No
Photon input from text file No No No No Yes
G4 optical photon tracking Yes Yes Yes No No
GPU simulation (Simphony) Yes Yes Yes Yes Yes
Multi-threaded Yes Yes No No No

GPUCerenkov and GPURaytrace collect gensteps from charged particle interactions and pass them to Simphony for GPU photon generation and tracing. GPUPhotonSource and GPUPhotonSourceMinimal instead generate photons directly from a torch configuration. GPUPhotonSource runs both G4 and GPU tracking for validation, while GPUPhotonSourceMinimal skips G4 tracking entirely to show the minimal GPU-only path. GPUPhotonFileSource reads photons from a user-provided text file, enabling custom photon distributions without code changes.

GDML scintillation properties for Geant4 11.x and Simphony

For scintillation to work with both Geant4 11.x and Simphony GPU simulation, the GDML must define properties using the correct syntax.

  1. Const properties, such as yield and time constants, must use coldim="1" matrices:
<define>
    <matrix coldim="1" name="SCINT_YIELD" values="5000.0"/>
    <matrix coldim="1" name="FAST_TIME_CONST" values="21.5"/>
</define>
  1. Both old and new style property names are required for Simphony compatibility:
<material name="Crystal">
    <!-- New Geant4 11.x names -->
    <property name="SCINTILLATIONYIELD" ref="SCINT_YIELD"/>
    <property name="SCINTILLATIONCOMPONENT1" ref="SCINT_SPECTRUM"/>
    <property name="SCINTILLATIONTIMECONSTANT1" ref="FAST_TIME_CONST"/>
    <!-- Old-style names for Opticks U4Scint -->
    <property name="FASTCOMPONENT" ref="SCINT_SPECTRUM"/>
    <property name="SLOWCOMPONENT" ref="SCINT_SPECTRUM"/>
    <property name="REEMISSIONPROB" ref="REEMISSION_PROB"/>
</material>

See tests/geom/8x8SiPM_w_CSI_optial_grease.gdml for a complete working example.

User and developer defined inputs

Defining primary particles

There are several inputs that the user or developer has to define. In src/GPUCerenkov, which imports src/GPUCerenkov.h, the code provides a working example with a simple geometry. The user or developer has to change the following details: the number of primary particles to simulate in a macro file and the number of G4 threads. For example:

/run/numberOfThreads {threads}
/run/verbose 1
/process/optical/cerenkov/setStackPhotons {flag}
/run/initialize
/run/beamOn 500

Here setStackPhotons defines whether G4 will propagate optical photons or not. In production, Simphony on the GPU takes care of optical photon propagation. Additionally, the user has to define the starting position, momentum, and related settings of the primary particles in the GeneratePrimaries function in src/GPUCerenkov.h. The hits of the optical photons are returned in the EndOfRunAction function. If more photons are simulated than can fit in GPU RAM, the execution of a GPU call should be moved to EndOfEventAction together with retrieving the hits.

Loading geometry into Simphony

Simphony can import geometries in GDML format automatically. About ten primitives are supported now, for example G4Box. G4Trd and G4Trap are not supported yet. GPUCerenkov takes GDML files through command line arguments, for example GPUCerenkov -g mygdml.gdml.

The GDML must define all optical properties of surfaces and materials, including:

  • Efficiency, used by Simphony to specify detection efficiency and assign sensitive surfaces
  • Refractive index
  • Group velocity
  • Reflectivity
  • Additional material and surface optical properties as required by the model