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## Re: Different radii for the 'same' electron in B-field

Keywords: GEATN4 magnetic field electrons curve
Forum: Fields: Magnetic and Otherwise
Re: Different radii for the 'same' electron in B-field (Valentin)
Re: Re: Different radii for the 'same' electron in B-field (John Apostolakis)
Date: 16 May, 2005
From: Valentin <Valentin.Kozlov@fys.kuleuven.ac.be>

 ```Thank you for your answer. Indeed when I switched to store position points in SteppingAction (via track->GetPosition().x() etc) the situation has changed: Since the electrons are produced in vacuum, in order to see the tracks I have to limit the step size (via UserLimits process). When it was done, I could see more or less only one circle of right diameter in XY plane (just to remind: e- start in direction (0,1,0), while B-field is (0,0,9)T). But.. in 3D(XYZ) it's a cylinder! and quite long cylinder (few 10s cm along Z). This means particle drifts in Z direction while it should not. The following was tried: 1) different EpsilonStep (1e-5 : 1e-7), no visible change 2) fMinStep for G4ChordFinder(), no visible change 3) stepMax (step size of track). this one is sensitive parameter: _stepMax=10mm_ : EkinInit[keV]=3000, p(0,1,0) Ekin[keV]=3000, m(e-)[keV]=510.999 p=sqrt{Ekin*(Ekin+2m(e-))}[keV]=3473.61, p[keV]=3473.61: p(-3.46044,0.302211,7.87798e-07) (-1.17539,1.28275,-323): 9 here: - first row is Initial Ekin and direction; - second row is Ekin in step, m(e-); - third row is momentum calculated via GetKineticEnergy() and via GetMomentum().x(): sqrt(px^2+py^2+pz^2), and momentum as (px,py,pz) - third row is (x,y,z) and field there. so, as one can see pz.neq.0 over sudden appears. _stepMax=1mm_ : EkinInit[keV]=3000, p(0,1,0) Ekin[keV]=3000, m(e-)[keV]=510.999 p=sqrt{Ekin*(Ekin+2m(e-))}[keV]=3473.61, p[keV]=3473.61: p(-2.43488,2.47737,6.43e-09) (-0.369229,0.902438,-323): 9 but... _stepMax=0.1mm_ : EkinInit[keV]=3000, p(0,1,0) Ekin[keV]=3000, m(e-)[keV]=510.999 p=sqrt{Ekin*(Ekin+2m(e-))}[keV]=3473.61, p[keV]=3473.61: p(-0.269542,3.46314,7.60947e-08) i.e. pz became larger than for stepMax=1mm; and if there is no "UserLimit": _no UserLimit_: EkinInit[keV]=3000, p(0,1,0) Ekin[keV]=3000, m(e-)[keV]=510.999 p=sqrt{Ekin*(Ekin+2m(e-))}[keV]=3473.61, p[keV]=3473.61: p(-2.8789,-1.94369,0) (-2.02679,1.09512,-323): 9 i.e. pz exactly equals zero, how it has to be! well, I think I am satisfied in my "research" but curious from where the inprecision comes from in case of UserLimit process? Best regards, Valentin```

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