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Question Stuck Track using very simple geometry and parallel beam model 

Forum: Geometry
Date: 24 Jul, 2016
From: Youming Yang <Youming Yang>

Hello,

I am getting a stuck track when using a simple voxelized model (using PVReplica) 1mm^3 voxels, and a G4GPS parallel beam of photons.

I am using Geantv4.10.01p02

The code for the geometry is:
    dx=dy=dz=1*mm;
    nx=ny=nz=401;
    detx=dx*nx;
    dety=dy*ny;
    detz=dz*nz;

    G4String detName("Det");
    G4VSolid* sDet = new G4Box(detName,detx/2.,dety/2.,detz/2.);
    lDet = new G4LogicalVolume(sDet, G4Water, "sDet", 0, 0, 0);
    new G4PVPlacement(0, G4ThreeVector(), lDet, "lDet", logicWorld, false,0,true);

    //XBOX
    G4String xName("xRep");
    G4VSolid* sXRep = new G4Box(xName,dx/2.,dety/2.,detz/2.);
    lXRep = new G4LogicalVolume(sXRep, G4Water, xName , 0, 0, 0);
    new G4PVReplica(xName,lXRep,lDet,kXAxis,nx,dx);

    //Y SLICE
    G4String yName("yRep");
    G4VSolid* sYRep = new G4Box(yName,dx/2.,dy/2.,detz/2.);
    G4LogicalVolume* lYRep =new G4LogicalVolume(sYRep,G4Water,yName);
    new G4PVReplica(yName,lYRep,lXRep,kYAxis,ny,dy);

    G4String zName("zRep");
    G4VSolid* sZRep = new G4Box(zName,dx/2.,dy/2.,dz/2.);
    G4LogicalVolume* lZRep =new G4LogicalVolume(sZRep,G4Water,zName);
    new G4PVReplica(zName,lZRep,lYRep,kZAxis,nz,dz);

And the input for G4GPS is:

    /gps/particle gamma

    /gps/pos/type Plane
    /gps/pos/shape Square
    /gps/pos/halfx 2.5 cm
    /gps/pos/halfy 2.5 cm

    /gps/ene/type Mono
    /gps/ene/mono 2 MeV
    /gps/pos/centre 0 0 -21 cm
    /gps/direction 0 0 1

I get "warning" messages such as:
    G4WT30 > 
    -------- WWWW ------- G4Exception-START -------- WWWW -------
    *** G4Exception : GeomNav1002
          issued by : G4Navigator::ComputeStep()
    Track stuck or not moving.
              Track stuck, not moving for 10 steps
              in volume -zRep- at point (-33.3568,-151.5,-64.5)
              direction: (0.020471,-0.879251,-0.475918).
              Potential geometry or navigation problem !
              Trying pushing it of 1e-07 mm ...Potential overlap in geometry!

    *** This is just a warning message. ***
    -------- WWWW -------- G4Exception-END --------- WWWW -------

    G4WT27 > 
    -------- WWWW ------- G4Exception-START -------- WWWW -------
    *** G4Exception : GeomNav1002
          issued by : G4Navigator::ComputeStep()
    Track stuck or not moving.
              Track stuck, not moving for 10 steps
              in volume -zRep- at point (-33.3568,-151.5,-64.5)
              direction: (0.020471,-0.879251,-0.475918).
              Potential geometry or navigation problem !
              Trying pushing it of 1e-07 mm ...Potential overlap in geometry!

    *** This is just a warning message. ***
    -------- WWWW -------- G4Exception-END --------- WWWW -------

    G4WT21 > 
    -------- WWWW ------- G4Exception-START -------- WWWW -------
    *** G4Exception : GeomNav1002
          issued by : G4Navigator::ComputeStep()
    Track stuck or not moving.
              Track stuck, not moving for 10 steps
              in volume -zRep- at point (-6.92026,30.5,-13.5)
              direction: (0.531012,-0.777328,-0.337325).
              Potential geometry or navigation problem !
              Trying pushing it of 1e-07 mm ...Potential overlap in geometry!

    *** This is just a warning message. ***
    -------- WWWW -------- G4Exception-END --------- WWWW -------

This issue only occurs when I use a photon beam (changing particle type to protons shows no issues, and the simulation completes successfully. However my photon simulation has been running for an order of magnitude longer without any output. I am under the impression it may be due to the parallel photon beams having initial trajectories collinear with voxel boundaries.

Does anyone have experience/suggestions for this situation?

Thank you, Ming

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1 More: Re: Stuck Track using very simple geometry and parallel beam model   (Gabriele Cosmo - 19 Aug, 2016)
2 None: Re: Stuck Track using very simple geometry and parallel beam model   (John Apostolakis - 19 Aug, 2016)
1 None: Re: Stuck Track using very simple geometry and parallel beam model   (Youming Yang - 20 Aug, 2016)
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