add post-step momentum direction to Steps ntuple

Pre-step direction was already recorded; post_dx/dy/dz lets students
compare momentum direction before and after each step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -153,6 +153,7 @@ One row per Geant4 step. Every step of every track of every event is recorded.
 | `layer_id` | `int` | Layer index at the pre-step point (-1 = outside all layers) |
 | `material` | `string` | Material name at the pre-step point |
 | `pre_dx/dy/dz` | `double` | Pre-step momentum unit direction |
+| `post_dx/dy/dz` | `double` | Post-step momentum unit direction |
 | `Bx/By/Bz` | `double` | Magnetic field at pre-step position [T] |
 | `Ex/Ey/Ez` | `double` | Electric field at pre-step position [V/m] |
 | `child_track_ids` | `int[]` | Track IDs of all secondaries spawned during this step (empty if none) |

src/RunAction.cc

@@ -111,6 +111,9 @@ RunAction::RunAction()
   analysisManager->CreateNtupleDColumn("Ey");          // col 24 [V/m]
   analysisManager->CreateNtupleDColumn("Ez");          // col 25 [V/m]
   analysisManager->CreateNtupleIColumn("child_track_ids", stepChildIds); // col 26 (variable-length array)
+  analysisManager->CreateNtupleDColumn("post_dx");     // col 27
+  analysisManager->CreateNtupleDColumn("post_dy");     // col 28
+  analysisManager->CreateNtupleDColumn("post_dz");     // col 29
   analysisManager->FinishNtuple();
 
   // Spawning ntuple (one row per secondary born, ntuple id=2)

src/SteppingAction.cc

@@ -143,6 +143,12 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
   analysisManager->FillNtupleDColumn(1, 18, dir.y());
   analysisManager->FillNtupleDColumn(1, 19, dir.z());
 
+  // G: post-step momentum direction (unit vector)
+  const auto postDir = post->GetMomentumDirection();
+  analysisManager->FillNtupleDColumn(1, 27, postDir.x());
+  analysisManager->FillNtupleDColumn(1, 28, postDir.y());
+  analysisManager->FillNtupleDColumn(1, 29, postDir.z());
+
   // Field: B [T] and E [V/m] at pre-step position; zero if no field is registered
   G4double Bx=0, By=0, Bz=0, Ex=0, Ey=0, Ez=0;
   const auto* fm = G4TransportationManager::GetTransportationManager()->GetFieldManager();