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Question Step time calculation 

Forum: Event and Track Management
Date: 27 Aug, 2013
From: Benjamin F <Benjamin F>

Hello, I am finding problems with calculating the step time in G4. I generate a primary particle (37K +1 ion) with essentially zero initial velocity (10^-5 mm/ns) and watch it propagate in a uniform electric field before striking a detector and recording the local time (or global time).

Given the step length, final and initial velocities it should be simple to calculate the step time (and this is confirmed in the Geant Physics Reference Manual).

http://geant4.web.cern.ch/geant4/G4UsersDocuments/UsersGuides/PhysicsReferenceManual/html/node21.html

I have tried various electric field stepper types but none work satisfactorily.

Below is the output when I turn on tracking/verbose/3. You can see that the expected time difference based on the formula in the reference manual is 468.4 ns and the reported time is 470.3 ns. (This is just one event; other events are similarly off but not by exactly the same ratio or amount).

The stepper here used is G4CashKarpRKF45.

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

    ++G4Step Information 
      Address of G4Track    : 0x1a7f310
      Step Length (mm)      : 10
      Energy Deposit (MeV)  : 1.187306976999772e-11
      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         Position - x (mm)   :                    00.002694155277345721
         Position - y (mm)   :                    0   9.999934290837558
         Position - z (mm)   :                    0-0.002315248395075974
         Global Time (ns)    :                    0   470.3252800447956
         Local Time (ns)     :                    0   470.3252800447956
         Proper Time (ns)    :                    0   470.3252800447954
         Momentum Direct - x :   0.72757967933865550.0001333171207950369
         Momentum Direct - y :  -0.2822887763974083  0.9999999845504328
         Momentum Direct - z :  -0.6252526344874632-0.0001145673571795582
         Momentum - x (MeV/c): 0.00065797370979303850.0006579736987469881
         Momentum - y (MeV/c): -0.0002552828215432984   4.935402779911176
         Momentum - z (MeV/c): -0.000565436071339341-0.0005654360618468107
         Total Energy (MeV)  :    34439.98428241977   34439.98463605234
         Kinetic Energy (MeV): 1.187306976999772e-110.000353632583392034
         Velocity (mm/ns)    : 7.872011835876924e-06 0.04296159156586455
         Volume Name         :           world_phys          world_phys
         Safety (mm)         :                    00.002250170179225464
         Polarization - x    :                    0                   0
         Polarization - y    :                    0                   0
         Polarization - Z    :                    0                   0
         Weight              :                    1                   1
         Step Status         :            Undefined       PostStep Proc
         Process defined Step:            Undefined         StepLimiter
      -----------------------------------------------------------------------

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

 Thanks in advance for your help!

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