I am observing a weird result when looking at tracking details within
two concentric tori (toruses?).

The following results are from Geant4 version 7.1, but I get the same
using pre-release version 7.1.ref03.

This is the code I use in DetectorConstruction to create the concentric tori:

@--------------------------------------------------------@
  //--------- Definitions of Solids, Logical Volumes, Physical Volumes --------

  //------------------------------
  // World
  //------------------------------

  G4double HalfWorldLength = 50.*m;
  G4Box* solidWorld =
    new G4Box("world",HalfWorldLength,HalfWorldLength,HalfWorldLength);
  G4LogicalVolume* logicWorld =
    new G4LogicalVolume(solidWorld,Vacuum,"World",0,0,0);
  logicWorld->SetVisAttributes(G4VisAttributes::Invisible);
  physiWorld = new G4PVPlacement(0,0,logicWorld,"World",0,false,0);

  //-------------------------
  // Toroidal fiber
  //-------------------------

  G4Torus* solidFiber =
    new G4Torus("clad",0.,1.*cm,10.*cm,0.*deg,180.*deg);
  G4LogicalVolume* logicFiber =
    new G4LogicalVolume(solidFiber,Glue,"FiberClad",0,0,0);
  new G4PVPlacement(0,0,logicFiber,"FiberClad",logicWorld,false,0);

  //-------------------------
  // Toroidal fiber core
  //-------------------------

  G4Torus* solidCore =
    new G4Torus("core",0.,0.5*cm,10.*cm,0.*deg,180.*deg);
  G4LogicalVolume* logicCore =
    new G4LogicalVolume(solidCore,Scintillator,"FiberCore",0,0,0);
  new G4PVPlacement(0,0,logicCore,"FiberCore",logicFiber,false,0);
@-------------------------------------------------------------@

I then shoot an up-going geantino from 0,0,0 with

@-------------------------------------------------------------@
  G4ParticleTable* particleTable = G4ParticleTable::GetParticleTable();
  G4ParticleDefinition* particle = particleTable->FindParticle("geantino");
  particleGun->SetParticleDefinition(particle);
  particleGun->SetParticlePosition(G4ThreeVector(0.*cm,0.*cm,0.*cm));
  particleGun->SetParticleMomentumDirection(G4ThreeVector(0.,1.,0.));
  particleGun->SetParticleEnergy(100.*MeV);
@-------------------------------------------------------------@

If I now look at tracking output, this is what I see:

@-------------------------------------------------------------@
#Step# 0
--- Pre-Step status
          World     0 fm       0 fm       0 fm
--- Post-Step status
          World     0 fm       0 fm       0 fm
--- Step quantities
Step Length  0 fm
Track Length 0 fm
Process      initStep

#Step# 1
--- Pre-Step status
          World     0 fm       0 fm       0 fm
--- Post-Step status
      FiberClad     0 fm       9 cm       0 fm
--- Step quantities
Step Length  9 cm
Track Length 9 cm
Process Transportation

#Step# 2
--- Pre-Step status
      FiberClad     0 fm       9 cm       0 fm
--- Post-Step status
      FiberCore     0 fm     9.5 cm       0 fm
--- Step quantities
Step Length  5 mm
Track Length 9.5 cm
Process Transportation

#Step# 3
--- Pre-Step status
      FiberCore     0 fm     9.5 cm       0 fm
--- Post-Step status
          World     0 fm    10.5 cm       0 fm
--- Step quantities
Step Length  1 cm
Track Length 10.5 cm
Process Transportation

#Step# 4
--- Pre-Step status
          World     0 fm    10.5 cm       0 fm
--- Post-Step status
      FiberClad     0 fm      11 cm       0 fm
--- Step quantities
Step Length  5 mm
Track Length 11 cm
Process Transportation

#Step# 5
--- Pre-Step status
      FiberClad     0 fm      11 cm       0 fm
--- Post-Step status
          World     0 fm      11 cm       0 fm
--- Step quantities
Step Length  0 fm
Track Length 11 cm
Process Transportation

#Step# 6
--- Pre-Step status
          World     0 fm      11 cm       0 fm
--- Post-Step status
     OutOfWorld     0 fm      50 m        0 fm
--- Step quantities
Step Length  49.9 m
Track Length 50 m
Process Transportation
@-------------------------------------------------------------@

I am worried about the 0-length step number 5 and about the fact that
step number 3 ends in the "World" volume rather than the expected
"FiberClad" (or "FiberCore"?).

I am not sure if this is a problem, but I would like to understand why
this happens.

Thanks

            Emanuele Leonardi