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Question Electrons bouncing on the volume boundary in pure magnetic field 

Keywords: Magnetic field, electrons, ions, tracking
Forum: Fields: Magnetic and Otherwise
Date: 20 Feb, 2006
From: Ilya <ilya.kraev@fys.kuleuven.ac.be>

(GEANT 4.7.0.p1)

Dear GEANTers,

I have encountered a problem with tracking of the electrons in magnetic field in a simple set-up. I have a source of electrons (radioactive 60Co) located in 20 mum thick foil which is made of VACUUM. Mother volume - WORLD (also vacuum) Electron emitted in a random direction hits the boundary of the foil and gets reflected into the opposite direction. It can even leave the volume and then it is reflected again into oposite direction in vacuum at some point (that does not have any specific properties) in the WORLD volume. It continues to loop like this for a huge number of steps and then it finaly leaves the volume and reaches the detector The same happens to the recoil ion.

The energy of electron is typical beta-spectrum with end-point energy of 320 keV. Recoil ion: 60Ni (energy is about several eV). I'm using the low-energy physics processes.

The processes defined for electron: G4MultipleScattering (with FacRange = 0.01) G4LowEnergyIonisation G4LowEnergyBremsstrahlung

For ion: G4MultipleScattering G4ionIonisation

The magnetic field is user-defined: I have a field profile with 13T central field and decreasing to approx. 1T over 20 cm. The field does not have any sharp points in profile. In the field definition I customize following parameters:

////////////////////////////////////////////////
fEquation = new G4Mag_UsualEqRhs(this);
fMinStep     = 0.01*mm; // minimal step of 1 mm is default
fMaxStep = 500.*mm;
fStepperType = 4 ;      // ClassicalRK4 is default stepper
fFieldManager = G4TransportationManager::GetTransportationManager()
                                      ->GetFieldManager();
G4TransportationManager::GetTransportationManager()->GetPropagatorInField()->SetMaxLoopCount(500);
G4TransportationManager::GetTransportationManager()->GetPropagatorInField()->SetLargestAcceptableStep(fMaxStep);
//////////////////////////////////////////////////

Please can somebody point out the problem (why it bounces like this) and possible solution (it takes ages to simulate few events or sometimes it stucks forever)!!!

Thanks a lot, Ilya.

Below is a part of output of tracking for one event:

*********************************************************************************************************
* G4Track Information:   Particle = Co60[0.0],   Track ID = 1,   Parent ID = 0
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0   -620 mum -1.18 mm   7.08 mum     0 eV      0 eV      0 fm      0 fm         Foil    initStep
    1   -620 mum -1.18 mm   7.08 mum     0 eV      0 eV      0 fm      0 fm         Foil  RadioactiveDecay
    :----- List of 2ndaries - #SpawnInStep=  5(Rest= 5,Along= 0,Post= 0), #SpawnTotal=  5 ---------------
    :   -620 mum -1.18 mm   7.08 mum 0.959 eV  Ni60[0.0]
    :   -620 mum -1.18 mm   7.08 mum  1.33 MeV     gamma
    :   -620 mum -1.18 mm   7.08 mum  1.17 MeV     gamma
    :   -620 mum -1.18 mm   7.08 mum   263 keV anti_nu_e
    :   -620 mum -1.18 mm   7.08 mum  55.4 keV        e-
    :----------------------------------------------------------------- EndOf2ndaries Info ---------------
Track (trackID 1, parentID 0) is processed with stopping code 2
A new track 0x928f760 (trackID 2, parentID 1) is passed to G4StackManager.
A new track 0x928f808 (trackID 3, parentID 1) is passed to G4StackManager.
A new track 0x928f8b0 (trackID 4, parentID 1) is passed to G4StackManager.
A new track 0x928f610 (trackID 5, parentID 1) is passed to G4StackManager.
A new track 0x9291030 (trackID 6, parentID 1) is passed to G4StackManager.
### pop requested out of 5 stacked tracks.
Selected G4StackedTrack : 0x928fa68 with G4Track 0x9291030 (trackID 6, parentID 1)
Track 0x9291030 (trackID 6, parentID 1) is passed to G4TrackingManager.

*********************************************************************************************************
* G4Track Information:   Particle = e-,   Track ID = 6,   Parent ID = 1
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0   -620 mum -1.18 mm   7.08 mum  55.4 keV     0 eV      0 fm      0 fm         Foil    initStep
    1   -611 mum -1.18 mm     10 mum  55.4 keV     0 eV   9.28 mum  9.28 mum        Foil  Transportation
    2   -671 mum  -1.1 mm     10 mum  55.4 keV     0 eV   49.3 cm   49.3 cm        World  Transportation
    3   -668 mum -1.15 mm    -10 mum  55.4 keV     0 eV   61.2 mum  49.3 cm         Foil  Transportation
    4   -561 mum -1.11 mm    -10 mum  55.4 keV     0 eV   48.9 cm   98.2 cm        World  Transportation
    5   -599 mum -1.07 mm     10 mum  55.4 keV     0 eV   59.4 mum  98.2 cm         Foil  Transportation
    6   -609 mum -1.18 mm     10 mum  55.4 keV     0 eV   48.1 cm   1.46 m         World  Transportation
    7   -566 mum -1.15 mm    -10 mum  55.4 keV     0 eV   57.7 mum  1.46 m          Foil  Transportation
    8   -639 mum -1.07 mm    -10 mum  55.4 keV     0 eV   47.7 cm   1.94 m         World  Transportation
    9   -674 mum -1.11 mm     10 mum  55.4 keV     0 eV   57.4 mum  1.94 m          Foil  Transportation
   10   -559 mum -1.12 mm     10 mum  55.4 keV     0 eV   47.4 cm   2.41 m         World  Transportation
   11   -583 mum -1.08 mm    -10 mum  55.4 keV     0 eV   55.7 mum  2.41 m          Foil  Transportation
   12   -619 mum -1.07 mm    -10 mum  55.4 keV     0 eV   46.8 cm   2.88 m         World  Transportation
   13   -662 mum -1.09 mm     10 mum  55.4 keV     0 eV   54.4 mum  2.88 m          Foil  Transportation
   14   -622 mum -1.18 mm     10 mum  55.4 keV     0 eV   46.8 cm   3.35 m         World  Transportation
   15   -576 mum -1.17 mm    -10 mum  55.4 keV     0 eV   53.6 mum  3.35 m          Foil  Transportation
   16   -667 mum  -1.1 mm    -10 mum  55.4 keV     0 eV   46.4 cm   3.81 m         World  Transportation
   17   -668 mum -1.15 mm     10 mum  55.4 keV     0 eV     53 mum  3.81 m          Foil  Transportation
   18   -578 mum -1.08 mm     10 mum  55.4 keV     0 eV     46 cm   4.27 m         World  Transportation
   19   -624 mum -1.07 mm    -10 mum  55.4 keV     0 eV   52.5 mum  4.27 m          Foil  Transportation
   20   -572 mum -1.16 mm    -10 mum  55.4 keV     0 eV   46.1 cm   4.74 m         World  Transportation
..................
  524    367 mum -1.32 mm     10 mum  55.4 keV     0 eV   46.6 cm    113 m         World  Transportation
  525    372 mum -1.32 mm    -10 mum  55.4 keV     0 eV   20.7 mum   113 m          Foil  Transportation
  526    300 mum -1.38 mm    -10 mum  55.4 keV     0 eV   37.3 cm    113 m         World  Transportation
  527    301 mum -1.38 mm     10 mum  55.4 keV     0 eV   20.4 mum   113 m          Foil  Transportation
  528   21.8 mum   660 mum  19.3 cm   55.4 keV     0 eV   20.7 cm    114 m         World  Transportation
  529   21.6 mum   660 mum  19.3 cm   54.9 keV   434 eV    292 nm    114 m    FrontDeadLayer         msc

HERE IT REACHED DETECTOR............................ (then it goes fine) next part is for ion

*********************************************************************************************************
* G4Track Information:   Particle = Ni60[0.0],   Track ID = 2,   Parent ID = 1
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0    590 mum   257 mum  5.63 mum  3.47 eV      0 eV      0 fm      0 fm         Foil    initStep
    1    593 mum   260 mum   -10 mum  3.47 eV      0 eV   29.9 mum  29.9 mum        Foil  Transportation
    2    593 mum   260 mum   -10 mum  3.47 eV 2.74e-16 eV   49.9 cm   49.9 cm        World  Transportation
    3    589 mum   256 mum    10 mum  3.47 eV      0 eV   42.4 mum  49.9 cm         Foil  Transportation
    4    588 mum   266 mum    10 mum  3.47 eV 2.73e-16 eV   49.7 cm   99.6 cm        World  Transportation
    5    592 mum   264 mum   -10 mum  3.47 eV      0 eV   41.8 mum  99.6 cm         Foil  Transportation
    6    586 mum   265 mum   -10 mum  3.47 eV 2.72e-16 eV   49.6 cm   1.49 m         World  Transportation
    7    592 mum   264 mum    10 mum  3.47 eV      0 eV   43.5 mum  1.49 m          Foil  Transportation
    8    588 mum   266 mum   797 mum  3.47 eV 2.74e-16 eV     50 cm   1.99 m         World  Transportation
    9    593 mum   261 mum    10 mum  3.47 eV 1.03e-18 eV   1.76 mm   1.99 m         World  Transportation
   10    589 mum   256 mum   -10 mum  3.47 eV      0 eV   44.9 mum  1.99 m          Foil  Transportation
   11    593 mum   261 mum -6.38 mm   3.47 eV 2.74e-16 eV     50 cm   2.49 m         World  Transportation
   12    585 mum   257 mum   -10 mum  3.47 eV 7.86e-18 eV   1.42 cm   2.51 m         World  Transportation
   13    584 mum   264 mum    10 mum  3.47 eV      0 eV   44.6 mum  2.51 m          Foil  Transportation
   14    593 mum   260 mum    10 mum  3.47 eV 2.71e-16 eV   49.4 cm      3 m         World  Transportation
   15    589 mum   256 mum   -10 mum  3.47 eV      0 eV   42.4 mum     3 m          Foil  Transportation
   16    589 mum   266 mum   -10 mum  3.47 eV 2.74e-16 eV   49.9 cm    3.5 m         World  Transportation
   17    594 mum   261 mum    10 mum  3.47 eV      0 eV   44.1 mum   3.5 m          Foil  Transportation
   18    588 mum   256 mum  4.26 mm   3.47 eV 2.74e-16 eV     50 cm      4 m         World  Transportation
   19    583 mum   261 mum    10 mum  3.47 eV 5.38e-18 eV   9.68 mm   4.01 m         World  Transportation
   20    589 mum   267 mum   -10 mum  3.47 eV      0 eV   45.6 mum  4.01 m          Foil  Transportation
   21    584 mum   259 mum -1.79 mm   3.47 eV 2.74e-16 eV     50 cm   4.51 m         World  Transportation
   22    588 mum   267 mum   -10 mum  3.47 eV 2.27e-18 eV   4.03 mm   4.51 m         World  Transportation
   23    593 mum   261 mum    10 mum  3.47 eV      0 eV   45.2 mum  4.51 m          Foil  Transportation
   24    584 mum   265 mum  1.59 mm   3.47 eV 2.74e-16 eV     50 cm   5.01 m         World  Transportation
   25    585 mum   266 mum    10 mum  3.47 eV 1.86e-18 eV   3.41 mm   5.02 m         World  Transportation
   26    591 mum   265 mum   -10 mum  3.47 eV      0 eV   43.1 mum  5.02 m          Foil  Transportation
   27    583 mum   261 mum -1.46 mm   3.47 eV 2.74e-16 eV     50 cm   5.52 m         World  Transportation
..........................
and so on

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1 Note: Re: Electrons bouncing on the volume boundary in pure magnetic field   (John Apostolakis - 07 Mar, 2006)
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to: "Electrons bouncing on the volume boundary in pure magnetic field"

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