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Forum: Geometry
Re: Question Stuck Track using very simple geometry and parallel beam model (Youming Yang)
Date: 19 Aug, 2016
From: John Apostolakis <John Apostolakis>

In addition to Gabriele's suggestions, I would propose:

- avoid photon directions which arrive exactly at the intersection of 
voxels.

    This is unphysical, as a photon beam has spread.

    Even if we are able to handle it for 99.99% of all cases, you do not 
want to have a problem in 1 in 10^4 or even in 1 in 10^6 initial particles.

- try to use the print 'modulo' feature to check how many tracks are 
making progress.  Is your problem happening in just one thread, which is 
not allowing the program to finish or in many (all?) threads.

- if you can isolate an event in which there is a problem, then please 
try to do verbose printing to see what type of problem is occuring.

Without further information it is very difficult to provide further 
suggestions.

Best regards,
  John


On 25/07/16 06:28, Youming Yang wrote:
> *** Discussion title: Geometry
>
> 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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