Editing Geant4 Detector Geometry (section)

===World Volume===

This <span style="color:red">'''world'''</span> volume must contain all other volumes within the detector geometry. Other volumes that are created, must be included inside the <span style="color:red">'''world'''</span> volume. The most simple and efficient shape to describe this <span style="color:red">'''world'''</span> volume would be a box.
Each volume is created by describing its shape and its physical characteristics, and then placing it inside a containing volume.
When a volume is placed within another volume, we call the former volume the daughter volume and the latter the mother volume. The coordinate system used to specify where the daughter volume is placed, is the coordinate system of the mother volume.

*To describe a volume's shape, we use the concept of a solid. A '''solid''' is a geometrical object that has a shape and specific values for each of that shape's dimensions. A cube with a side of 10 centimeters and a cylinder of radius 30 cm and length 75 cm are examples of solids.
*To describe a volume's full properties, we use a '''logical''' volume. It includes the geometrical properties of the solid, and adds physical characteristics: the material of the volume; whether it contains any sensitive detector elements; the magnetic field; etc.
*To position the volume. To do this you create a '''physical''' volume, which places a copy of the logical volume inside a larger, containing, volume.

Our code to create the <span style="color:red">'''World'''</span> volume:
 G4double world_sizeXY = 1.2*env_sizeXY;
  G4double world_sizeZ  = 1.2*env_sizeZ;
  G4Material* world_mat = nist->FindOrBuildMaterial("G4_AIR");
  
  G4Box* '''solid'''World =    
    new G4Box("World",                       //its name
       0.5*world_sizeXY, 0.5*world_sizeXY, 0.5*world_sizeZ);     //its size
      
  G4LogicalVolume* '''logic'''World =                         
    new G4LogicalVolume(solidWorld,          //its solid
                        world_mat,           //its material
                        "World");            //its name
                                   
  G4VPhysicalVolume* '''phys'''World = 
    new G4PVPlacement(0,                     //no rotation
                      G4ThreeVector(),       //at (0,0,0)
                      logicWorld,            //its logical volume
                      "World",               //its name
                      0,                     //its mother  volume
                      false,                 //no boolean operation
                      0,                     //copy number
                      checkOverlaps);        //overlaps checking