Editing Geant4 Detector Geometry (section)

===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]