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Sad electron stuck on volume boundary with (local) uniform magnetic field 

Forum: Fields: Magnetic and Otherwise
Date: 21 Jul, 2008
From: Brian Fisher <Brian Fisher>

I'm experiencing a situation similar to posting #96 in this forum. That thread seems to have gone dead, so I'm posting a new thread.

I'm using Geant4 9.1p02

In a very simple setup, electrons sometimes are "bouncing" between a volume (with a local magnetic field) and its mother (which does not have a magnetic field).  Both volumes are filled with "vacuum."

I get many (tens of thousands) of messages like these:

WARNING - G4Navigator::ComputeStep()
          Track stuck, not moving for 10 steps
          in volume -expHall- at point (-8.257550732,-9.52505,18.49195399)
          direction: (-0.01057780492,7.285534641e-05,0.9999440508).
          Potential geometry or navigation problem !
          Trying pushing it of 9e-10 mm ...

I've been modeling charged particle detector telescopes, and in the process of hunting this problem down, I have reduced the geometry of my problem to:

1) a 1x1x1 m^3 box filled with vacuum. This is the "expHall" volume
2) a "BFieldZone" box also filled with vacuum with the following parameters:

 Solid type: G4Box
 Parameters:  half length X: 1.39526 cm 
              half length Y: 0.952505 cm 
              half length Z: 0.952505 cm 
-----------------------------------------------------------
 Center of Box: X = 0 cm Y = 0 cm Z = 1.98525 cm 

3) the "BFieldZone" box has a uniform 2 kilogauss magnetic field in the +X direction.

This is how I've defined the (local) magnetic field for the "BFieldZone" volume:

	G4double = fFieldValue = 2.0*kilogauss;
	fMagField = new G4UniformMagField(G4ThreeVector(fFieldValue, 0.0, 0.0));
	fEquation = new G4Mag_UsualEqRhs(fMagField);
	fStepper = new G4ExactHelixStepper(fEquation);
	fIntgrDriver = new G4MagInt_Driver(1.0*micrometer, fStepper, fStepper->GetNumberOfVariables(), 2);
	fChordFinder = new G4ChordFinder(fIntgrDriver);
	fFieldMgr = new G4FieldManager(fMagField, fChordFinder);

	logicBFieldZone->SetFieldManager(fFieldMgr, true);

4) The "expHall" box has no magnetic field.


When I turn the tracking verbosity up to 2, the 'offending' event looks like this:


*********************************************************************************************************
* G4Track Information:   Particle = e-,   Track ID = 1,   Parent ID = 0
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0  -8.06 mm  -9.69 mm 7.87e-55 fm   1.97 MeV     0 eV      0 fm      0 fm      expHall    initStep
    1  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV4.82e-19 eV   1.81 cm   1.81 cm      expHall  Transportation
    2  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm   physiBFieldZone  Transportation
    3  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm      expHall  Transportation
    4  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm   physiBFieldZone  Transportation
    5  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm      expHall  Transportation
    6  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm   physiBFieldZone  Transportation
    7  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm      expHall  Transportation
    8  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm   physiBFieldZone  Transportation
    9  -8.25 mm  -9.53 mm   1.81 cm   1.97 MeV     0 eV      0 fm   1.81 cm      expHall  Transportation
   10  -8.25 mm  -9.52 mm   1.82 cm   1.97 MeV2.39e-21 eV   89.9 um   1.82 cm   physiBFieldZone  Transportation
   11  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV1.91e-22 eV   7.16 um   1.82 cm   physiBFieldZone  Transportation
   12  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm      expHall  Transportation
   13  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm   physiBFieldZone  Transportation
   14  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm      expHall  Transportation
   15  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm   physiBFieldZone  Transportation
   16  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm      expHall  Transportation
   17  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm   physiBFieldZone  Transportation
   18  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm      expHall  Transportation
   19  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm   physiBFieldZone  Transportation
   20  -8.25 mm  -9.53 mm   1.82 cm   1.97 MeV     0 eV      0 fm   1.82 cm      expHall  Transportation
   21  -8.26 mm  -9.52 mm   1.83 cm   1.97 MeV2.39e-21 eV   89.9 um   1.83 cm   physiBFieldZone  Transportation
   22  -8.26 mm  -9.53 mm   1.83 cm   1.97 MeV1.32e-22 eV   4.96 um   1.83 cm   physiBFieldZone  Transportation
   23  -8.26 mm  -9.53 mm   1.83 cm   1.97 MeV     0 eV      0 fm   1.83 cm      expHall  Transportation
   24  -8.26 mm  -9.53 mm   1.83 cm   1.97 MeV     0 eV      0 fm   1.83 cm   physiBFieldZone  Transportation
   25  -8.26 mm  -9.53 mm   1.83 cm   1.97 MeV     0 eV      0 fm   1.83 cm      expHall  Transportation
   26  -8.26 mm  -9.53 mm   1.83 cm   1.97 MeV     0 eV      0 fm   1.83 cm   physiBFieldZone  Transportation

[and so on, seemingly forever]


If the verbosity is turned up to 3, I get this:

[snip]

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
30132  -8.26 mm  -9.53 mm   1.85 cm   1.97 MeV     0 eV      0 fm   1.85 cm   physiBFieldZone  Transportation

 >>AlongStepDoIt (after all invocations):
    ++List of invoked processes 
      1) Transportation
      2) msc
      3) eIoni
      4) eBrem

    ++G4Step Information 
      Address of G4Track    : 0x3046970
      Step Length (mm)      : 0
      Energy Deposit (MeV)  : 0
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -8.257551022101929  -8.257551022101929
         Position - y (mm)   :             -9.52505            -9.52505
         Position - z (mm)   :    18.49198141201282   18.49198141201282
         Global Time (ns)    :  0.06304591361302052 0.06304591361302052
         Local Time (ns)     :  0.06304591361302052 0.06304591361302052
         Proper Time (ns)    :  0.01300913309755812 0.01300913309755812
         Momentum Direct - x : -0.01057780492354854-0.01057780492354854
         Momentum Direct - y : 7.285445476253925e-057.285445476253925e-05
         Momentum Direct - z :   0.9999440508024575  0.9999440508024575
         Momentum - x (MeV/c): -0.02563161644372119-0.02563161644372119
         Momentum - y (MeV/c): 0.00017653733021041490.0001765373302104149
         Momentum - z (MeV/c):    2.423015224859268   2.423015224859268
         Total Energy (MeV)  :    2.476444998409542   2.476444998409542
         Kinetic Energy (MeV):    1.965445938409542   1.965445938409542
         Velocity (mm/ns)    :    293.3407906754122   293.3407906754122
         Volume Name         :              expHall             expHall
         Safety (mm)         :                5e-10               5e-10
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :           Geom Limit          Geom Limit
         Process defined Step:       Transportation      Transportation
      -----------------------------------------------------------------------

    ++List of secondaries generated (x,y,z,kE,t,PID):  No. of secodaries = 0

 **PostStepDoIt (after all invocations):
    ++List of invoked processes 
      1) Transportation
      2) msc
      3) eIoni (Forced)

    ++G4Step Information 
      Address of G4Track    : 0x3046970
      Step Length (mm)      : 0
      Energy Deposit (MeV)  : 0
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -8.257551022101929  -8.257551022101929
         Position - y (mm)   :             -9.52505            -9.52505
         Position - z (mm)   :    18.49198141201282   18.49198141201282
         Global Time (ns)    :  0.06304591361302052 0.06304591361302052
         Local Time (ns)     :  0.06304591361302052 0.06304591361302052
         Proper Time (ns)    :  0.01300913309755812 0.01300913309755812
         Momentum Direct - x : -0.01057780492354854-0.01057780492354854
         Momentum Direct - y : 7.285445476253925e-057.285445476253925e-05
         Momentum Direct - z :   0.9999440508024575  0.9999440508024575
         Momentum - x (MeV/c): -0.02563161644372119-0.02563161644372119
         Momentum - y (MeV/c): 0.00017653733021041490.0001765373302104149
         Momentum - z (MeV/c):    2.423015224859268   2.423015224859268
         Total Energy (MeV)  :    2.476444998409542   2.476444998409542
         Kinetic Energy (MeV):    1.965445938409542   1.965445938409542
         Velocity (mm/ns)    :    293.3407906754122   293.3407906754122
         Volume Name         :              expHall     physiBFieldZone
         Safety (mm)         :                5e-10               5e-10
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :           Geom Limit          Geom Limit
         Process defined Step:       Transportation      Transportation
      -----------------------------------------------------------------------

    ++List of secondaries generated (x,y,z,kE,t,PID):  No. of secodaries = 0
      [Note]Secondaries from AlongStepDoIt included.

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
30133  -8.26 mm  -9.53 mm   1.85 cm   1.97 MeV     0 eV      0 fm   1.85 cm      expHall  Transportation

 >>AlongStepDoIt (after all invocations):
    ++List of invoked processes 
      1) Transportation
      2) msc
      3) eIoni
      4) eBrem

    ++G4Step Information 
      Address of G4Track    : 0x3046970
      Step Length (mm)      : 0
      Energy Deposit (MeV)  : 0
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -8.257551022101929  -8.257551022101929
         Position - y (mm)   :             -9.52505            -9.52505
         Position - z (mm)   :    18.49198141201282   18.49198141201282
         Global Time (ns)    :  0.06304591361302052 0.06304591361302052
         Local Time (ns)     :  0.06304591361302052 0.06304591361302052
         Proper Time (ns)    :  0.01300913309755812 0.01300913309755812
         Momentum Direct - x : -0.01057780492354854-0.01057780492354854
         Momentum Direct - y : 7.285445476253925e-057.285445476253925e-05
         Momentum Direct - z :   0.9999440508024575  0.9999440508024575
         Momentum - x (MeV/c): -0.02563161644372119-0.02563161644372119
         Momentum - y (MeV/c): 0.00017653733021041490.0001765373302104149
         Momentum - z (MeV/c):    2.423015224859268   2.423015224859268
         Total Energy (MeV)  :    2.476444998409542   2.476444998409542
         Kinetic Energy (MeV):    1.965445938409542   1.965445938409542
         Velocity (mm/ns)    :    293.3407906754122   293.3407906754122
         Volume Name         :      physiBFieldZone     physiBFieldZone
         Safety (mm)         :                5e-10               5e-10
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :           Geom Limit          Geom Limit
         Process defined Step:       Transportation      Transportation
      -----------------------------------------------------------------------

    ++List of secondaries generated (x,y,z,kE,t,PID):  No. of secodaries = 0

 **PostStepDoIt (after all invocations):
    ++List of invoked processes 
      1) Transportation
      2) msc
      3) eIoni (Forced)

    ++G4Step Information 
      Address of G4Track    : 0x3046970
      Step Length (mm)      : 0
      Energy Deposit (MeV)  : 0
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -8.257551022101929  -8.257551022101929
         Position - y (mm)   :             -9.52505            -9.52505
         Position - z (mm)   :    18.49198141201282   18.49198141201282
         Global Time (ns)    :  0.06304591361302052 0.06304591361302052
         Local Time (ns)     :  0.06304591361302052 0.06304591361302052
         Proper Time (ns)    :  0.01300913309755812 0.01300913309755812
         Momentum Direct - x : -0.01057780492354854-0.01057780492354854
         Momentum Direct - y : 7.285445476253925e-057.285445476253925e-05
         Momentum Direct - z :   0.9999440508024575  0.9999440508024575
         Momentum - x (MeV/c): -0.02563161644372119-0.02563161644372119
         Momentum - y (MeV/c): 0.00017653733021041490.0001765373302104149
         Momentum - z (MeV/c):    2.423015224859268   2.423015224859268
         Total Energy (MeV)  :    2.476444998409542   2.476444998409542
         Kinetic Energy (MeV):    1.965445938409542   1.965445938409542
         Velocity (mm/ns)    :    293.3407906754122   293.3407906754122
         Volume Name         :      physiBFieldZone             expHall
         Safety (mm)         :                5e-10               5e-10
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :           Geom Limit          Geom Limit
         Process defined Step:       Transportation      Transportation
      -----------------------------------------------------------------------

    ++List of secondaries generated (x,y,z,kE,t,PID):  No. of secodaries = 0
      [Note]Secondaries from AlongStepDoIt included.

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
30134  -8.26 mm  -9.53 mm   1.85 cm   1.97 MeV     0 eV      0 fm   1.85 cm   physiBFieldZone  Transportation


[and so on... ]

So I'm wondering why is my step length at the boundary being set to zero?  
Do I have a bad set of physics lists?  
Is the fact that this particle is nearly perpendicular to the field the problem? 

Any help anyone can provide is GREATLY appreciated.

Thanks,

Brian

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1 Feedback: Re: electron stuck on volume boundary with (local) uniform magnetic field   (Gumplinger Peter - 21 Jul, 2008)
(_ More: Re: electron stuck on volume boundary with (local) uniform magnetic field   (Brian Fisher - 21 Jul, 2008)
(_ More: Re: electron stuck on volume boundary with (local) uniform magnetic field   (Brian Fisher - 21 Jul, 2008)
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