Difference GEANT3-GEANT4 via VMC: media in magnetic field (CBMROOT)

[kisselan]

Hello.
We found some difference in particle acceptance using GEANT3 and 4. After some study I found possible source of difference.
I simulated proton and anti-p with flat momentum distribution. They fly through magnetic field until detector. I built momentum of the particles accepted in the detector. These distributions in GEANT3 and 4 are different for particles with momentum lower 2-3 GeV/c. I changed the medium description for GEANT3, and can reproduce GEANT4 distribution. I tried to explain my study a few slides. I would like to ask: could you please check, that all medium parameters concerning magnetic field are proper calculated or converted from input file to the GEANT3, and they are the same as for GEANT4.

Best regards
Anna Senger
G3_G4.pdf

[ihrivnac]

Hello @kisselan ,

In Geant4 VMC, only some of tracking media parameters are taken into account.
Can you, please, explain the meaning of the tracking media parameters that you put on your slides, such as

// ----- Vacuum ----------------------------------------------------------
Vacuum  1 1.e-16 1.e-16 1.e-16
        0 1 20. .001
        0

They seem to be FairRoot specific.

Thank you,

[kisselan]

The all explanations you can find here: https://fairroot.gsi.de/index.html%3Fq=node%252F34.html

[ihrivnac]

Hello @kisselan ,

Sorry for a delay; I confirm that the four parameters for field defined for a tracking medium: sensflag, fldflag, fld and epsil are not taken into account by Geant4 by default. It is possible to choose to take into account the sensflag with

/mcDet/setIsZeroMagField true 

There are also commands available for customisation of parameters for field propagation described in Geant4 VMC documentation:

https://vmc-project.github.io/user-guide/geant4_vmc/magnetic-field/

Best regards,