In nature, general materials (chemical compounds, mixtures) are made of elements, and elements are made of isotopes. Therefore, these are the three main classes designed in Geant4. Each of these classes has a table as a static data member, which is for keeping track of the instances created of the respective classes.
The G4Element class describes the properties of the atoms:
number of nucleons,
as well as quantities such as cross sections per atom, etc.
The G4Material class describes the macroscopic properties of matter:
as well as macroscopic quantities like radiation length, mean free path, dE/dx, etc.
The G4Material class is the one which is visible to the rest of the toolkit, and is used by the tracking, the geometry, and the physics. It contains all the information relative to the eventual elements and isotopes of which it is made, at the same time hiding the implementation details.
In the example below, liquid argon is created, by specifying its name, density, mass per mole, and atomic number.
Example 2.7. Creating liquid argon.
G4double density = 1.390*g/cm3; G4double a = 39.95*g/mole; G4Material* lAr = new G4Material(name="liquidArgon", z=18., a, density);
The pointer to the material, lAr, will be used to specify the matter of which a given logical volume is made:
G4LogicalVolume* myLbox = new G4LogicalVolume(aBox,lAr,"Lbox",0,0,0);
In the example below, the water, H2O, is built from its components, by specifying the number of atoms in the molecule.
Example 2.8. Creating water by defining its molecular components.
a = 1.01*g/mole; G4Element* elH = new G4Element(name="Hydrogen",symbol="H" , z= 1., a); a = 16.00*g/mole; G4Element* elO = new G4Element(name="Oxygen" ,symbol="O" , z= 8., a); density = 1.000*g/cm3; G4Material* H2O = new G4Material(name="Water",density,ncomponents=2); H2O->AddElement(elH, natoms=2); H2O->AddElement(elO, natoms=1);
In the example below, air is built from nitrogen and oxygen, by giving the fractional mass of each component.
Example 2.9. Creating air by defining the fractional mass of its components.
a = 14.01*g/mole; G4Element* elN = new G4Element(name="Nitrogen",symbol="N" , z= 7., a); a = 16.00*g/mole; G4Element* elO = new G4Element(name="Oxygen" ,symbol="O" , z= 8., a); density = 1.290*mg/cm3; G4Material* Air = new G4Material(name="Air ",density,ncomponents=2); Air->AddElement(elN, fractionmass=70*perCent); Air->AddElement(elO, fractionmass=30*perCent);
In the example below, air and water are accessed via the Geant4 material database.
Example 2.10. Defining air and water from the internal Geant4 database.
G4NistManager* man = G4NistManager::Instance(); G4Material* H2O = man->FindOrBuildMaterial("G4_WATER"); G4Material* Air = man->FindOrBuildMaterial("G4_AIR");
Example 2.11. Printing information about materials.
G4cout << H2O; \\ print a given material G4cout << *(G4Material::GetMaterialTable()); \\ print the list of materials
will find examples of all possible ways to build a material.