a wrong angular distribution of alpha ?

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a wrong angular distribution of alpha ?

Hi:
   I have made a simulation on the reaction B10(n,alpha)Li7 with Geant4.9.2. The energy of neutron is 0.0253ev.
  But in my simulation I got a angular distribution of alpha which is not 
isotropic.
   My physicsList is LHEP_PRECO_HP.
   My SteppingAction as follows:

    const G4Track* track = step->GetTrack();
    G4String name = track->GetDefinition()->GetParticleName();
   if(name == "alpha"){
    G4ThreeVector dir = track->GetVertexMomentumDirection(); 
    G4ThreeVector pos = track->GetVertexPosition();
    G4cout<<pos.z()<<G4endl; 
    G4cout<<dir.z()<<G4endl;
    G4double dir_z = dir.z();
    G4double cosPhi = dir_z; 
    G4int Phi=int(acos(cosPhi)/deg); 
    NumID2[Phi]+=1;
  }

  Will you give me some help?
   I think my physicsList is right,but the SteppingAction may be wrong.So what's the ringt SteppingAction?

Re: a wrong angular distribution of alpha ? by Marcus Mendenhall <Marcus Mendenhall>, 18 Apr, 2009 Re: a wrong angular distribution of alpha ? You appear to be histogramming in phi, but you should get a flat distribution histogramming cos(phi). Are you later correcting for this when you check for isotropy? Your distribution should look like sin(phi) if you are collecting it in uniform width phi bins, because of the d(cos(phi)) which comes into the solid angle.

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