G4VSolid* slabIN= new G4Box("slabIN",14*cm,14*cm,14.375*cm);
     G4VSolid* slabOUT = new G4Box("slabOUT",15*cm,15*cm,15*cm);
     G4VSolid* slabph= new G4SubtractionSolid("slabPhantom", slabOUT, slabIN,
0,G4ThreeVector(0,0,-0.375*cm));

    G4LogicalVolume* logicslabph =
    new G4LogicalVolume(slabph,            //its solid
                        pmma_mat,             //its material
                        "slabph");         //its name

  new G4PVPlacement(0,                       //no rotation
                    G4ThreeVector(0,0,15.365*cm),         //at (0,0,0)
                    logicslabph,                //its logical volume
                    "slabph",              //its name
                    logicEnv,              //its mother  volume
                    true,                   //no boolean operation
                    0,                       //copy number
                    checkOverlaps);          //overlaps checking
  
  G4VisAttributes* slabphVisAtt= new G4VisAttributes(G4Colour(1.0,0.0,0.0));
  logicslabph->SetVisAttributes(slabphVisAtt); 
     
     
      G4Material* water = nist->FindOrBuildMaterial("G4_WATER");
    
  G4LogicalVolume* logicwater =
    new G4LogicalVolume(slabIN,            //its solid
                        water,             //its material
                        "water");         //its name

  new G4PVPlacement(0,                       //no rotation
                    G4ThreeVector(0,0,14.990*cm),         //at (0,0,0)
                    logicwater,                //its logical volume
                    "water",              //its name
                    logicEnv,              //its mother  volume
                    false,                   //no boolean operation
                    0,                       //copy number
                    checkOverlaps);          //overlaps checking