Materials Example

This example demonstrates the four ways to define a material in GEMC: using a Geant4 built-in material, composing a material from elements by atom count, composing a material from existing materials by fractional mass, and adding optical properties for scintillation or Cherenkov radiation.

You can run this example in your browser:

Quickstart

Copy the example to your current directory. To create the geometry and run 10 events:

cp -r $GEMC_HOME/examples/basic/material .
cd material
./materials.py
gemc materials.yaml -n=10

Geometry

The geometry is defined in materials.py.

The world (a box named root) contains five identical tubes placed along the z-axis. All tubes share the same shape: 10 cm diameter, 1 cm thick, starting at z = 0 with a 3 cm gap between them. They differ only in color and material definition:

Interactive viewer:

Material Definitions

Tube 1 — Geant4 built-in material

The simplest case: assign a name from the Geant4 Material Database directly to gvolume.material:

tube1.material = "G4_C"

No GMaterial object is needed; GEMC resolves the name at run time.

Tube 2 — Molecular composition

Use GMaterial and addNAtoms to specify atoms and their count in the chemical formula:

water = GMaterial("custom_water")
water.description = "Water defined by molecular composition (H2O)"
water.density = 1.0          # g/cm3
water.addNAtoms("H", 2)
water.addNAtoms("O", 1)
water.publish(cfg)

The total atom count must be greater than 1.

Tube 3 — Fractional masses

Use addMaterialWithFractionalMass to mix existing materials by weight fraction. The fractions must sum to exactly 1:

mixture = GMaterial("air_water_mixture")
mixture.description = "80% air / 20% water mixture"
mixture.density = 0.9601     # g/cm3
mixture.addMaterialWithFractionalMass("G4_AIR",   0.80)
mixture.addMaterialWithFractionalMass("G4_WATER", 0.20)
mixture.publish(cfg)

Tube 4 — Scintillation properties

Set photonEnergy and the scintillation fields to enable optical photon emission. All energy-dependent quantities must have the same number of entries as photonEnergy:

scintillator = GMaterial("my_scintillator")
scintillator.density = 3.67   # g/cm3
scintillator.addNAtoms("Na", 1)
scintillator.addNAtoms("I",  1)
scintillator.photonEnergy       = "2.0*eV 2.5*eV 3.0*eV 3.5*eV 4.0*eV"
scintillator.fastcomponent      = "1.0 0.9 0.8 0.7 0.6"
scintillator.slowcomponent      = "0.5 0.4 0.3 0.2 0.1"
scintillator.scintillationyield = 38000   # photons/MeV
scintillator.resolutionscale    = 1.0
scintillator.fasttimeconstant   = 6       # ns
scintillator.slowtimeconstant   = 88      # ns
scintillator.yieldratio         = 0.8
scintillator.birksConstant      = 0.00152
scintillator.publish(cfg)

Tube 5 — Index of refraction (Cherenkov)

Set photonEnergy, indexOfRefraction, and optionally absorptionLength to enable Cherenkov radiation in the material. This mirrors the approach used in the Cherenkov example:

optical_glass = GMaterial("optical_glass")
optical_glass.density = 2.5   # g/cm3
optical_glass.addNAtoms("Si", 1)
optical_glass.addNAtoms("O",  2)
optical_glass.photonEnergy      = "2.0*eV 3.0*eV 4.0*eV 5.0*eV"
optical_glass.indexOfRefraction = "1.458 1.466 1.476 1.490"
optical_glass.absorptionLength  = "3*m 3*m 3*m 3*m"
optical_glass.publish(cfg)

Physics List

FTFP_BERT + G4OpticalPhysics is selected in the YAML file so that optical photons from scintillation and Cherenkov radiation are tracked.

phys_list: FTFP_BERT + G4OpticalPhysics

phys_list

The physics list can be selected using the option

gemc -phys_list <value>

where <value> can be a combination of the Geant4 physics constructors separated by the + sign. For example

gemc -phys_list="FTFP_BERT + G4NeutronCrossSectionXS"

To see a list of the available Geant4 constructors:

gemc -showPhysics

Generator

The particle kinematics are defined in the YAML file:

gparticle:
  - name: proton
    p: 2000*MeV
    vz: -2*cm
    delta_vx: 0.1*cm
    delta_vy: 0.1*cm
    multiplicity: 1

See also the Internal Generator Documentation for more information.

Usage

Building the detector

./materials.py

See also the Building Geometry for more information.

Running GEMC

gemc materials.yaml -n=10

Scene annotations and decorations are kept in annotations.yaml. Run the main YAML by itself for an uncluttered view, or pass both YAML files to include the labels and scale decoration:

gemc materials.yaml -n=10
gemc materials.yaml annotations.yaml -n=10

Add -gui to run interactively.

Running Events

Output

Two output streams are configured in the YAML file:

gstreamer:
  - format: csv
    filename: material

See also the Output Documentation for more information.