add child_track_ids array column to Steps; remove parent_id

Each Steps row now carries a variable-length child_track_ids int array
containing the track IDs of all secondaries spawned during that step.
The parent_id column is removed — parentage is now expressed
unidirectionally (parent→children) via child_track_ids and the Spawning
ntuple, rather than bidirectionally.

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

include/RunAction.hh

@@ -83,6 +83,8 @@ class RunAction : public G4UserRunAction
   mutable std::vector<int> hitLayer;
   mutable std::vector<int> hitCopyNumber;
 
+  mutable std::vector<int> stepChildIds;
+
   mutable G4String filename;
 
 };

include/SteppingAction.hh

@@ -33,6 +33,7 @@
 #include "G4UserSteppingAction.hh"
 #include "GeometryDescriptor.hh"
 #include "DetectorConstruction.hh"
+#include "RunAction.hh"
 
 
 namespace B4a
@@ -54,8 +55,11 @@ public:
 
   void UserSteppingAction(const G4Step* step) override;
 
+  void setRunAction(const B4::RunAction* runAction) { fRunAction = runAction; }
+
 private:
   EventAction* fEventAction = nullptr;
+  const B4::RunAction* fRunAction = nullptr;
 };
 
 }

src/ActionInitialization.cc

@@ -70,6 +70,7 @@ void ActionInitialization::Build() const
   eventAction->setPrimaryGeneratorAction(gen);
   SetUserAction(eventAction);
   auto steppingAction = new SteppingAction(eventAction);
+  steppingAction->setRunAction(runact);
   SetUserAction(steppingAction);
 }
 

src/RunAction.cc

@@ -98,19 +98,19 @@ RunAction::RunAction()
   analysisManager->CreateNtupleDColumn("post_E");      // col 11
   analysisManager->CreateNtupleDColumn("edep");        // col 12 [MeV]
   analysisManager->CreateNtupleDColumn("step_length"); // col 13 [mm]
-  analysisManager->CreateNtupleIColumn("parent_id");   // col 14
-  analysisManager->CreateNtupleSColumn("process");     // col 15
-  analysisManager->CreateNtupleIColumn("layer_id");    // col 16  (-1 = outside all layers)
-  analysisManager->CreateNtupleSColumn("material");    // col 17
-  analysisManager->CreateNtupleDColumn("pre_dx");      // col 18
-  analysisManager->CreateNtupleDColumn("pre_dy");      // col 19
-  analysisManager->CreateNtupleDColumn("pre_dz");      // col 20
-  analysisManager->CreateNtupleDColumn("Bx");          // col 21 [T]
-  analysisManager->CreateNtupleDColumn("By");          // col 22 [T]
-  analysisManager->CreateNtupleDColumn("Bz");          // col 23 [T]
-  analysisManager->CreateNtupleDColumn("Ex");          // col 24 [V/m]
-  analysisManager->CreateNtupleDColumn("Ey");          // col 25 [V/m]
-  analysisManager->CreateNtupleDColumn("Ez");          // col 26 [V/m]
+  analysisManager->CreateNtupleSColumn("process");     // col 14
+  analysisManager->CreateNtupleIColumn("layer_id");    // col 15  (-1 = outside all layers)
+  analysisManager->CreateNtupleSColumn("material");    // col 16
+  analysisManager->CreateNtupleDColumn("pre_dx");      // col 17
+  analysisManager->CreateNtupleDColumn("pre_dy");      // col 18
+  analysisManager->CreateNtupleDColumn("pre_dz");      // col 19
+  analysisManager->CreateNtupleDColumn("Bx");          // col 20 [T]
+  analysisManager->CreateNtupleDColumn("By");          // col 21 [T]
+  analysisManager->CreateNtupleDColumn("Bz");          // col 22 [T]
+  analysisManager->CreateNtupleDColumn("Ex");          // col 23 [V/m]
+  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->FinishNtuple();
 
   // Spawning ntuple (one row per secondary born, ntuple id=2)

src/SteppingAction.cc

@@ -87,10 +87,13 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
   auto track = step->GetTrack();
   int evtId  = G4RunManager::GetRunManager()->GetCurrentEvent()->GetEventID();
 
-  // Write one Spawning row per secondary born in this step (track IDs pre-assigned)
+  // Write one Spawning row per secondary born in this step (track IDs pre-assigned);
+  // also populate stepChildIds for the child_track_ids vector column in Steps.
+  fRunAction->stepChildIds.clear();
   const auto* secondaries = step->GetSecondaryInCurrentStep();
   if (secondaries) {
     for (const G4Track* sec : *secondaries) {
+      fRunAction->stepChildIds.push_back(sec->GetTrackID());
       analysisManager->FillNtupleIColumn(2, 0, evtId);
       analysisManager->FillNtupleIColumn(2, 1, track->GetTrackID());
       analysisManager->FillNtupleIColumn(2, 2, track->GetCurrentStepNumber());
@@ -118,14 +121,11 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
   // B: geometric step length
   analysisManager->FillNtupleDColumn(1, 13, step->GetStepLength());
 
-  // C: parent track ID (0 for primary)
-  analysisManager->FillNtupleIColumn(1, 14, track->GetParentID());
-
   // D: process that ended this step
   G4String processName = "";
   const auto* postProc = post->GetProcessDefinedStep();
   if (postProc) processName = postProc->GetProcessName();
-  analysisManager->FillNtupleSColumn(1, 15, processName);
+  analysisManager->FillNtupleSColumn(1, 14, processName);
 
   // E: layer index (-1 if outside all layers) and material name at pre-step point
   int layerId = -1;
@@ -134,14 +134,14 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
     if (layers[i].physicalVolume == volume) { layerId = i; break; }
   }
   G4String materialName = pre->GetMaterial() ? pre->GetMaterial()->GetName() : "";
-  analysisManager->FillNtupleIColumn(1, 16, layerId);
-  analysisManager->FillNtupleSColumn(1, 17, materialName);
+  analysisManager->FillNtupleIColumn(1, 15, layerId);
+  analysisManager->FillNtupleSColumn(1, 16, materialName);
 
   // F: pre-step momentum direction (unit vector)
   const auto dir = pre->GetMomentumDirection();
-  analysisManager->FillNtupleDColumn(1, 18, dir.x());
-  analysisManager->FillNtupleDColumn(1, 19, dir.y());
-  analysisManager->FillNtupleDColumn(1, 20, dir.z());
+  analysisManager->FillNtupleDColumn(1, 17, dir.x());
+  analysisManager->FillNtupleDColumn(1, 18, dir.y());
+  analysisManager->FillNtupleDColumn(1, 19, dir.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;
@@ -161,12 +161,13 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
       Ez = fieldVal[5] / (volt/m);
     }
   }
-  analysisManager->FillNtupleDColumn(1, 21, Bx);
-  analysisManager->FillNtupleDColumn(1, 22, By);
-  analysisManager->FillNtupleDColumn(1, 23, Bz);
-  analysisManager->FillNtupleDColumn(1, 24, Ex);
-  analysisManager->FillNtupleDColumn(1, 25, Ey);
-  analysisManager->FillNtupleDColumn(1, 26, Ez);
+  analysisManager->FillNtupleDColumn(1, 20, Bx);
+  analysisManager->FillNtupleDColumn(1, 21, By);
+  analysisManager->FillNtupleDColumn(1, 22, Bz);
+  analysisManager->FillNtupleDColumn(1, 23, Ex);
+  analysisManager->FillNtupleDColumn(1, 24, Ey);
+  analysisManager->FillNtupleDColumn(1, 25, Ez);
+  // col 26 child_track_ids: vector column, auto-read from fRunAction->stepChildIds at AddNtupleRow
 
   analysisManager->AddNtupleRow(1);
 }