Editing Geant4 Detector Geometry

A detector geometry in Geant4 is made of a number of volumes. The largest of these volumes is called the <span style="color:red">'''world'''</span> volume.

===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

===The Envelope (The world within the world)===

The <span style="color:blue">'''Envelope'''</span> is a smaller volume that exists within the <span style="color:red">'''world'''</span> volume. While the <span style="color:red">'''world'''</span> volume is created to house all inner volumes, the <span style="color:blue">'''envelope'''</span> is where the experiment occurs. The <span style="color:blue">'''envelope'''</span> can be thought of as a box within a box, as it is essentially a smaller box within the <span style="color:red">'''world'''</span> box. Within the <span style="color:blue">'''envelope'''</span>, other volumes (including materials) can be placed, and particles can be allowed to pass through. Although the volumes are described as a box, they can be other shapes as well, such as a cylinder. The code for creating boxes and cylinders can be found [http://geant4.cern.ch/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/ch02s02.html#sect.HowToDefDetectorGeom.ChooseSolid here].

The code for creating the <span style="color:blue">'''envelope'''</span> is near identical to the code for creating the <span style="color:red">'''world'''</span>. The only difference is that the volume is given a different name, and the dimensions should be small enough to fit inside the <span style="color:red">'''world'''</span>. Inside the <span style="color:blue">'''envelope'''</span>, other volumes and even materials with varying properties can be placed. This will be discussed in the next section.

===Creating the Geometry inside the Envelope===
Inside the <span style="color:blue">'''envelope'''</span> you can conduct many types of experiments. For our experiment we have placed two scoring volumes inside our <span style="color:blue">'''envelope'''</span>. Remember, each volume is created by describing its shape and its physical characteristics, and then placing it inside a containing volume; Please Refer to [http://jlabdaq.pcs.cnu.edu/wiki/index.php/Program_Structure#The_Envelope_.28The_world_within_the_world.29 The Envelope (The world within the world)]
A sample code of someone creating the <span style="color:blue">'''envelope'''</span>:
'''A shape'''
In this section of code we are defining the shape in our chosen coordinate system, which is Cartesian. Follow the comments throughout the code for a better understanding:
 G4Material* shape1_mat = nist->FindOrBuildMaterial("MATERIAL");
  G4ThreeVector pos1 = G4ThreeVector(x, y ,z);
  // whatever shape you want     
  G4double shape1_dya = cm, shape1_dyb = cm;
  G4double shape1_dxa = cm, shape1_dxb = cm;
  G4double shape1_dz  = cm;      
  G4Trd* '''solid'''Shape1 =    
    new G4Trd("Shape1",                      //its name
              0.5*shape1_dxa, 0.5*shape1_dxb, 
              0.5*shape1_dya, 0.5*shape1_dyb, 0.5*shape1_dz); //its size
                
  G4LogicalVolume* '''logic'''Shape1 =                         
    new G4LogicalVolume('''solid'''Shape1,         //its solid
                        shape1_mat,          //its material
                        "Shape1");           //its name
               
  new G4PVPlacement(0,                       //no rotation
                    pos1,                    //at position
                    logicShape1,             //its logical volume
                    "Shape1",                //its name
                    logicEnv,                //its mother  volume
                    false,                   //no boolean operation
                    0,                       //copy number
                    checkOverlaps);          //overlaps checking
                
 
Next is the creation of the scintillators

===How to Specify the Material in the Detector Geometry===
So within the <span style="color:blue">'''Envelope'''</span> we constructed these volumes, volumes however can be filled with a material. Geant4 does a great job in allowing the user extreme customization. I say this because, I can literally fill up these volumes with whatever I would like as long as it is in the ''G4Material'' class. 
The ''G4Material'' class describes the macroscopic properties of matter:
*density,
*state,
*temperature,
*pressure,
*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.
We defined our volumes within our <span style="color:blue">'''envelope'''</span> and then went on to tell the program what material we would like in each volume.
This is the section of code we used:
 G4Material* shape1_mat = nist->FindOrBuildMaterial("G4_POLYETHYLENE");
 G4Material* shape2_mat = nist->FindOrBuildMaterial("G4_POLYETHYLENE");
 //"nist" is calling the data from the National Institute of Technology
This is a list of elements, compounds, space ISS materials, bio-chemical materials, HEP, and nuclear materials [http://geant4.cern.ch/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/apas08.html here]