The particle erroneously propagated to the mother volume in a current step if

in the previous step particle exited from daughter  (G4Navigator fExiting=true)

and

in the current step G4VEnergyLossProcess::PostStepGetPhysicalInteractionLength()  returned 0   [ Most probably any process with PostStepGetPhysicalInteractionLengh() returned 0 will lead to same behaviour]



in that case

G4Transportation::AlongStepGetPhysicalInteractionLength  will not clear G4Navigator::fExiting(because of currentMinimumStep==0)  and will set  fGeometryLimitedStep=true inspite currentMinimumStep=0  /because it is on boundary(?) ,ie currentSafety==0/


G4Transportation::PostStepDoIt will call  G4Navigator::LocateGlobalPointAndSetup  which  will reduce fHistory depth /as fExiting=true/   and erroneously move particle to the mother volume.


Then one step in mother volume will be done , which  lead to miss corresponding underlying volumes.  After this step navigation resules in a normal way/




What is affected:  Any charged particles but in particularly ions with high energy (>TeV/nuc) , which naturally emit a lot of delta rays


Versions >=  4.9.6 including 4.10.01.p01 are affected.


Versions <= 4.9.4 were not affected because  G4VEnergyLossProcess::PostStepGetPhysicalInteractionLength()  never returned 0.

So trivial workarond would be just reinstate this behaviour.

The fix may be as simple as to clear fExiting in G4Transportation::AlongStepGetPhysicalInteractionLength even for  currentMinimumStep==0, however I don;t think I am qualified enough to say more.