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Question Re: Scintillation Process invoked in Non Scintillators 

Forum: Processes Involving Optical Photons
Re: None Scintillation Process invoked in Non Scintillators (Marcus O'Flaherty)
Re: Feedback Re: Scintillation Process invoked in Non Scintillators (Gumplinger Peter)
Date: 10 Jun, 2016
From: Marcus O'Flaherty <Marcus O'Flaherty>

Hi Peter,
Thanks for taking the time to explain this, I had a feeling something of that sort may be going on under the hood. I'm still a little confused though.
If scintillation is forced at every step but does nothing (so should not kill a track), why do some particles have a final ProcessDefinedStep of 'Transportation', while others have 'Scintillation' - even in the case that the final step was not in a Scintillator?

To clarify:
In sitations where the step length is limited by transportation, Scintillation will still be invoked (it's always invoked).
If the fact that this process is invoked (even though it didn't do anything) also makes it override 'Transportation' as a ProcessDefinedStep, (even though Transportation is what actually limited the step length) I would expect not to see 'Transportation' at all. Particles would leave the world volume, but on the way out would have scintillation forced, and so report that as their final process.

On the other hand, if Scintillation does not override transportation as the ProcessDefinedStep, a step limited by 'Transportation' will report as such. In that case, I would expect never to see Scintillation as a particle's final step in a non-scintillator. In any case where scintillation limits the step length, the process will not do anything, and so should not stop Geant4 tracking the particle. The particle will continue to be tracked until it leaves by Transportation, and then will report as such.

The only other case I can think of is if the step length is limited to DBL_MAX by Scintillation, but some other (forced? unreported?) process results in an energy loss which puts the particle below a tracking threshold. Geant stops tracking it as a result of that energy loss process, but doesn't report the process that actually changed it's state. Which seems a bit sneaky.

As suggested I cranked up verbosity on tracking and this is what I see:

*********************************************************************************************************
* G4Track Information:   Particle = proton,   Track ID = 478,   Parent ID = 3
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0  -1.32 m    3.04 cm  -58.3 cm   1.19 MeV     0 eV      0 fm      0 fm    waterTank    initStep
    1  -1.32 m    3.04 cm  -58.3 cm      0 eV   1.19 MeV  30.9 um   30.9 um    waterTank       hIoni
    2  -1.32 m    3.04 cm  -58.3 cm      0 eV      0 eV      0 fm   30.9 um    waterTank  Scintillation
SCINTILLATION IN waterTank
material is Water
energy deposited: 0 eV 
position change: (0 fm , 0 fm , 0 fm )
step length: 0 fm 

*********************************************************************************************************
* G4Track Information:   Particle = proton,   Track ID = 474,   Parent ID = 3
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0  -1.32 m    3.04 cm  -58.3 cm     12 MeV     0 eV      0 fm      0 fm    waterTank    initStep
    1  -1.32 m    3.05 cm  -58.2 cm   9.92 MeV  2.06 MeV   489 um    489 um    waterTank       hIoni
    2  -1.32 m    3.06 cm  -58.2 cm   7.99 MeV  1.93 MeV   391 um    880 um    waterTank       hIoni
    3  -1.32 m    3.06 cm  -58.1 cm    6.1 MeV  1.89 MeV   312 um   1.19 mm    waterTank       hIoni
    4  -1.32 m    3.07 cm  -58.1 cm   4.24 MeV  1.86 MeV   244 um   1.44 mm    waterTank       hIoni
    5  -1.32 m    3.07 cm  -58.1 cm   2.18 MeV  2.06 MeV   183 um   1.62 mm    waterTank       hIoni
    6  -1.32 m    3.07 cm  -58.1 cm      0 eV   2.18 MeV  84.7 um    1.7 mm    waterTank       hIoni
    7  -1.32 m    3.07 cm  -58.1 cm      0 eV      0 eV      0 fm    1.7 mm    waterTank  Scintillation
SCINTILLATION IN waterTank
material is Water
energy deposited: 0 eV 
position change: (0 fm , 0 fm , 0 fm )
step length: 0 fm 

There's some ionisation happening, followed by Scintillation in what looks like a somewhat pointless step - the particle doesn't move or lose energy - and after that Geant stops tracking it. I find it strange that 2 subsequent steps are performed when the KE is already 0. Even more puzzling: I tried more verbosity:
*********************************************************************************************************
* G4Track Information:   Particle = e-,   Track ID = 4143,   Parent ID = 2050
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    3  -1.07 m     -31 cm  -16.9 cm    182 keV     0 eV      0 fm   4.58 mm    waterTank    initStep

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

      3) eIoni


    ++G4Step Information 
      Address of G4Track    : 0x18695cc0
      Step Length (mm)      : 0.3830200779505322
      Energy Deposit (MeV)  : 0.1822999190961176
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -1069.605657146713  -1069.749674099378
         Position - y (mm)   :   -309.9155573756946  -310.0591833729748
         Position - z (mm)   :     -169.43609068196  -169.4179063788585
         Global Time (ns)    :    12.04498545178903   12.04599336342903
         Local Time (ns)     :  0.01370721478441643 0.01471512642441721
         Proper Time (ns)    : 0.004964546813305195 0.00570743246059266
         Momentum Direct - x :  -0.7052542321759863 -0.7052542321759863
         Momentum Direct - y :  -0.7033397149304729 -0.7033397149304729
         Momentum Direct - z :  0.08904893822714083 0.08904893822714083
         Momentum - x (MeV/c):  -0.3304500899885768                  -0
         Momentum - y (MeV/c):  -0.3295530341933742                  -0
         Momentum - z (MeV/c):  0.04172428651686985                   0
         Total Energy (MeV)  :   0.6932988290961176          0.51099891
         Kinetic Energy (MeV):   0.1822999190961176                   0
         Velocity (mm/ns)    :    202.6028057766495                   0
         Volume Name         :            waterTank           waterTank
         Safety (mm)         :                    0   440.8531131378492
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :            Undefined     AlongStep Proc.
         Process defined Step:            Undefined               eIoni
      -----------------------------------------------------------------------

    ++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) eBrem (Forced)

    ++G4Step Information 
      Address of G4Track    : 0x18695cc0
      Step Length (mm)      : 0.3830200779505322
      Energy Deposit (MeV)  : 0.1822999190961176
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -1069.605657146713  -1069.749674099378
         Position - y (mm)   :   -309.9155573756946  -310.0591833729748
         Position - z (mm)   :     -169.43609068196  -169.4179063788585
         Global Time (ns)    :    12.04498545178903   12.04599336342903
         Local Time (ns)     :  0.01370721478441643 0.01471512642441721
         Proper Time (ns)    : 0.004964546813305195 0.00570743246059266
         Momentum Direct - x :  -0.7052542321759863 -0.7052542321759863
         Momentum Direct - y :  -0.7033397149304729 -0.7033397149304729
         Momentum Direct - z :  0.08904893822714083 0.08904893822714083
         Momentum - x (MeV/c):  -0.3304500899885768                  -0
         Momentum - y (MeV/c):  -0.3295530341933742                  -0
         Momentum - z (MeV/c):  0.04172428651686985                   0
         Total Energy (MeV)  :   0.6932988290961176          0.51099891
         Kinetic Energy (MeV):   0.1822999190961176                   0
         Velocity (mm/ns)    :    202.6028057766495                   0
         Volume Name         :            waterTank           waterTank
         Safety (mm)         :                    0   440.8531131378492
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :            Undefined     AlongStep Proc.
         Process defined Step:            Undefined               eIoni
      -----------------------------------------------------------------------

    ++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
    4  -1.07 m     -31 cm  -16.9 cm      0 eV    182 keV   383 um   4.96 mm    waterTank       eIoni
 **List of AtRestDoIt invoked:
   # 1 : Scintillation (Forced)
   Generated secondries # : 0

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    5  -1.07 m     -31 cm  -16.9 cm      0 eV      0 eV      0 fm   4.96 mm    waterTank  Scintillation
SCINTILLATION IN waterTank
material is Water
energy deposited: 0 eV 
position change: (0 fm , 0 fm , 0 fm )
step length: 0 fm

===============================================================================================
===============================================================================================

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
   12  -1.05 m   -53.5 cm  -41.5 cm    206 keV   865 eV   3.09 um   9.19 mm    waterTank    Cerenkov

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

    ++G4Step Information 
      Address of G4Track    : 0x18693dd0
      Step Length (mm)      : 0.4661486174206785
      Energy Deposit (MeV)  : 0.2060976486627001
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -1046.487761397869  -1046.722560394046
         Position - y (mm)   :   -535.4441333559715  -535.3659991090209
         Position - z (mm)   :   -414.7165288035816  -414.6861771836837
         Global Time (ns)    :     13.1563816601031   13.15756706773492
         Local Time (ns)     :  0.02910938727426184 0.03029479490608251
         Proper Time (ns)    : 0.009280312574971699  0.0101250272796353
         Momentum Direct - x :  -0.9417856888994325 -0.9417856888994325
         Momentum Direct - y :   0.3133987657068557  0.3133987657068557
         Momentum Direct - z :   0.1217412413179729  0.1217412413179729
         Momentum - x (MeV/c):  -0.4738104879222266                  -0
         Momentum - y (MeV/c):   0.1576702893705209                   0
         Momentum - z (MeV/c):  0.06124777391397145                   0
         Total Energy (MeV)  :   0.7170965586627001          0.51099891
         Kinetic Energy (MeV):   0.2060976486627001                   0
         Velocity (mm/ns)    :    210.3179936683398                   0
         Volume Name         :            waterTank           waterTank
         Safety (mm)         :    398.3326119098709   398.0835812284361
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :        PostStep Proc     AlongStep Proc.
         Process defined Step:             Cerenkov               eIoni
      -----------------------------------------------------------------------

    ++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) eBrem (Forced)

    ++G4Step Information 
      Address of G4Track    : 0x18693dd0
      Step Length (mm)      : 0.4661486174206785
      Energy Deposit (MeV)  : 0.2060976486627001
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :   -1046.487761397869  -1046.722560394046
         Position - y (mm)   :   -535.4441333559715  -535.3659991090209
         Position - z (mm)   :   -414.7165288035816  -414.6861771836837
         Global Time (ns)    :     13.1563816601031   13.15756706773492
         Local Time (ns)     :  0.02910938727426184 0.03029479490608251
         Proper Time (ns)    : 0.009280312574971699  0.0101250272796353
         Momentum Direct - x :  -0.9417856888994325 -0.9417856888994325
         Momentum Direct - y :   0.3133987657068557  0.3133987657068557
         Momentum Direct - z :   0.1217412413179729  0.1217412413179729
         Momentum - x (MeV/c):  -0.4738104879222266                  -0
         Momentum - y (MeV/c):   0.1576702893705209                   0
         Momentum - z (MeV/c):  0.06124777391397145                   0
         Total Energy (MeV)  :   0.7170965586627001          0.51099891
         Kinetic Energy (MeV):   0.2060976486627001                   0
         Velocity (mm/ns)    :    210.3179936683398                   0
         Volume Name         :            waterTank           waterTank
         Safety (mm)         :    398.3326119098709   398.0835812284361
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :        PostStep Proc     AlongStep Proc.
         Process defined Step:             Cerenkov               eIoni
      -----------------------------------------------------------------------

    ++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
   13  -1.05 m   -53.5 cm  -41.5 cm      0 eV    206 keV   466 um   9.65 mm    waterTank       eIoni
 **List of AtRestDoIt invoked:
   # 1 : Scintillation (Forced)
   Generated secondries # : 0

#Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
   14  -1.05 m   -53.5 cm  -41.5 cm      0 eV      0 eV      0 fm   9.65 mm    waterTank  Scintillation
SCINTILLATION IN waterTank
material is Water
energy deposited: 0 eV 
position change: (0 fm , 0 fm , 0 fm )
step length: 0 fm 

*********************************************************************************************************
I can only see "Scintillation (Forced)" in AtRestDoIt, which seems to contradict the idea that it forced in *every* step. It seems that after any number of preceding steps, which have no mention of scintillation, once the particle reaches 0 KE, there are two further steps: eIoni (or hIoni for protons etc) followed by Scintillation. Why do we have two separate steps with 0 energy? And why is Scintillation being invoked as a separate distinct step, even when it has 0 energy, and the medium isn't a scintillator? And why is it at this step that geant stops tracking?

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1 Feedback: Re: Scintillation Process invoked in Non Scintillators   (Gumplinger Peter - 10 Jun, 2016)
(_ None: Re: Scintillation Process invoked in Non Scintillators   (Marcus O'Flaherty - 11 Jun, 2016)
(_ Feedback: Re: Scintillation Process invoked in Non Scintillators   (Gumplinger Peter - 13 Jun, 2016)
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