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Question Times or positions don't seem correct in synchrotron production in atmosphere model 

Forum: Event and Track Management
Date: 28 Jan, 2009
From: Scott Nutter <Scott Nutter>

I have been trying to figure this out for months to no avail. It's time to consult the experts.

I have a model of the atmosphere using G4Orbs embedded like "Russian dolls" with equal depth (20 mg/cm2) between layers.  I have a model of the Earth's magnetic field, and set 
        myField = new CrestMagField(latitude, longitude, time);
        G4FieldManager* fieldMgr
            = G4TransportationManager::GetTransportationManager()
              ->GetFieldManager();
        fieldMgr->SetDetectorField(myField);
        fieldMgr->CreateChordFinder(myField);
after constructing all of my geometry.  Field strengths are ~0.5 gauss and electron energies are ~10 TeV.  A more detailed description of my geometry is in a recent posting in the Geometry section.

I record all synchrotron photon hit locations and times at a certain height. I should be able to look at two close together (within meters) photons and reconstruct their zenith angle using their time and position differences. However, the resulting zenith angle is always nearly vertical, meaning either the times are too close together or the positions are too far apart. I carefully tag photons when produced by synchrotron and not undergoing any other physics, such as Compton scattering, before getting to my recording height. So unless the Transportation process itself can change directions, these should have traveled in a straight line. I verified the photns and the primary electron travel at c_light.

The zenith angle reconstruction method works fine with an old "flat earth" model based on the old GEANT3 Runge-Kutta tracker, so that works fine.

The default values for the Chordfinder are

fieldMgr->GetDeltaIntersection(): 1 um

fieldMgr->GetDeltaOneStep(): 10 um

End of run statistics for one track for the chordfinder are
G4ChordFinder statistics report:
  No trials: 4842  No Calls: 4701  Max-trial: 2
  Parameters:   fFirstFraction 0.999  fFractionLast 1  fFractionNextEstimate 0.98

Any ideas? I don't know where else to pursue this.

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