KAHIBARO
8.2. Geant4 Material Database
Table of Contents
Standard materials
Geant4 provides a large internal library of predefined materials. GATE uses this library directly, so most of the time you do not need to define common materials yourself. These predefined entries are often called “standard materials” and they are created in Geant4 C++ code before your GATE simulation starts.
Each standard material has a unique string name. When you configure geometry in GATE, you refer to the material by this name. You do not specify the composition or density again. For example, you can write something like:
water_box.material = "G4_WATER"
air_world.material = "G4_AIR"
lead_shield.material = "G4_Pb"
Here, "G4_WATER", "G4_AIR" and "G4_Pb" are names of standard materials from the Geant4 database. Internally, Geant4 already knows the element composition, mass fraction, and density for each of these.
Standard materials cover many typical use cases, including air, water, vacuum, some metals, plastics, and biological tissues. A few very common names that you will see in GATE examples are:
| Category | Example Geant4 name | Typical use in GATE |
|---|---|---|
| Air / atmosphere | G4_AIR | World volume, room, outside the patient |
| Water | G4_WATER | Water phantoms, dosimetry benchmarks |
| Vacuum | G4_Galactic | Vacuum regions, beam lines in accelerators |
| Plastic | G4_PLASTIC_SC_VINYLTOLUENE | Scintillator, detector blocks |
| Aluminum | G4_Al | Mechanical housing, collimator structures |
| Copper | G4_Cu | Cables, electronics boards, components |
| Lead | G4_Pb | Radiation shielding, collimators |
These materials are defined with consistent physical data so that interaction cross sections and energy loss are handled correctly by Geant4 physics.
In practice, you only need to know two things about standard materials. First, you must use the exact material name that Geant4 expects. If you misspell it, GATE will report an unknown material error. Second, you should pick a predefined material whenever it matches your intended physical object, because this avoids manual mistakes in composition or density.
If you are not sure which standard material name to use, you can start from GATE example scripts, documentation, or the Geant4 material reference list. For many basic simulations, such as water phantoms or simple shielding, sticking to "G4_WATER", "G4_AIR", and "G4_Pb" is sufficient.
Always use the exact Geant4 material name, for example "G4_WATER" or "G4_AIR". A small typo creates an unknown material and the simulation will fail or behave incorrectly.
NIST materials
A large fraction of the Geant4 material database comes from data compiled by NIST, the National Institute of Standards and Technology. These are called “NIST materials” in Geant4. They provide reference compositions and densities that match widely used NIST datasets for stopping power and interaction cross sections.
In GATE, NIST materials appear as standard Geant4 materials with names that start with G4_ and correspond to elements or common compounds. The main difference is that these materials follow NIST reference definitions. For example, many pure elements are available as NIST materials:
silicon_sensor.material = "G4_Si"
gold_foil.material = "G4_Au"
tungsten_shield.material = "G4_W"In addition to pure elements, NIST also defines a number of mixtures and compounds, such as various plastics, biological tissues, and construction materials. For medical physics simulations, NIST-based definitions are particularly important because they are widely used to compute dose and attenuation. Examples include:
| NIST material name | Typical use in medical simulations |
|---|---|
G4_WATER | Reference medium for dosimetry, water phantoms |
G4_BONE_COMPACT_ICRU | Bone-equivalent material, CT and dosimetry phantoms |
G4_LUNG_ICRP | Lung tissue, anthropomorphic phantoms |
G4_ADIPOSE_TISSUE_ICRP | Fat tissue, patient body composition |
G4_MUSCLE_SKELETAL_ICRP | Skeletal muscle tissue |
G4_CONCRETE | Room walls, bunker shielding |
The exact list of NIST materials is part of Geant4 itself, not GATE, so you do not create them in Python. Instead, you simply refer to their names when you assign materials to volumes. GATE forwards the name to Geant4, which looks up the corresponding NIST material.
NIST materials are especially useful whenever you want realistic dose or attenuation in tissues. Using these predefined definitions helps you avoid errors in stoichiometric composition and density. It also makes your simulations easier to reproduce, because other users can run the same geometry and be confident that they are using the same material definitions.
When NIST offers a suitable tissue or compound, prefer the NIST material over a custom definition. Leave custom materials for special cases that are not covered by the database, such as proprietary detector crystals or unusual mixtures.
Views: 10