How to change the momentum direction of particle after the particle hit a specific volume

Particles, Track, Event, Run and Biasing
1

Dear everyon:
I want to change the momentum direction of particle after the particle hit a specific volume. I use the function: SetMomentumDirection(newMomentumDirection) in UserSteppingAction(). But this can only change the momentum direction for one step. How to change the momentum direction and make the particle maintain this momentum direction in the after step?

2 
if (volumename == "physicdirectionchanger")
{
  G4Track* theTrack = const_cast<G4Track*>(track);
  G4ThreeVector newMomentumDirection = G4ThreeVector(0,1,1);
  theTrack->SetMomentumDirection(newMomentumDirection);
}

This is the code i used.

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