Import Geant4 11.2.1 source tree

examples/basic/B2/B2a/src/PrimaryGeneratorAction.cc

@@ -88,9 +88,9 @@ void PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
     G4cerr << "The gun will be place in the center." << G4endl;
   }
 
-  // Note that this particular case of starting a primary particle on the world boundary
-  // requires shooting in a direction towards inside the world.
-  fParticleGun->SetParticlePosition(G4ThreeVector(0., 0., -worldZHalfLength));
+  // Starting a primary particle close to the world boundary.
+  //
+  fParticleGun->SetParticlePosition(G4ThreeVector(0., 0., -worldZHalfLength+1*um));
 
   fParticleGun->GeneratePrimaryVertex(anEvent);
 }