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Question Stepsize underflow and illegal momentum/energy/time 

Forum: Fields: Magnetic and Otherwise
Date: 18 Jan, 2018
From: Kate Svechnikova <Kate Svechnikova>

Hello,

I'm trying to simulate propagation of electrons with energy 10 keV - 1 MeV in air, with ambient electric field. The detector is the NaI box with aluminium cover. The field strength is about 5 keV/cm - 20 keV/cm. The problem is that for particles with energy 0.1 - 1 MeV in homogeneous electric field one particle from approximately each hundred causes one of the following two error messages:

1.

G4WT1 > G4MagIntegratorDriver::OneGoodStep:

G4WT1 > Stepsize underflow in Stepper

G4WT1 > Step's start x=0.285469 and end x= 0.285469 are equal !!

G4WT1 > Due to step-size= 6.03928e-18 . Note that input step was 0.021741

As far as I understood, this error has nothing to do with field discountinuity (homogenious electric field and zero magnetic field are applied). Volumes have no intersections.

2.

*** G4Exception : TRACK003

      issued by : G4ParticleChange::CheckIt

momentum, energy, and/or time was illegal

*** Event Must Be Aborted ***

This error occurs even in modeling with zero electric field. It looks completely mysterious for me, but the error appears much less frequently, if the value of /event/verbose is more than zero. From file G4ParticleChange.cc I see, that there are several possible reasons: itsOK = itsOKforMomentum && itsOKforEnergy && itsOKforVelocity && itsOKforProperTime && itsOKforGlobalTime - so, something is wrong with at least one of these values. However, programm output here isn't exhaustive enough: I see no additional messages concerning momentum//energy//time.

If someone has any suggestions about the reasons of the errors or about the possible solutions, it will be highly appreciated.

Best regards, Kate

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