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Forum: Event and Track Management
Re: Question Precision of calculations in Geant4 (Maciej Sznajder)
Date: 09 Dec, 2010
From: Marc Verderi <Marc Verderi>

Hello Maciej,

     Could you tell more about what you would like to do with your 
simulation, what is your goal ? When you speak about good result, what 
quantity do you look at ?

     I believe that your problem should be better posted to the 
electromagnetic physics forum.

     Some elements of answer. I believe that what you call "event" is 
indeed "step" in the Geant4 meaning, is this correct ? If so, that fact 
you see many steps with G4CoulombScattering before the proton stops is 
perfectly normal : with G4CoulombScattering, which is intended for very 
low density materials, you make diffusion atom per atom. There is the 
most precise way, but of course the most expensive one in term of 
computing ! Multiple scattering is precisely done for avoiding this : 
having a model that integrates many single Coulomb interactions so that 
the particle is making macroscopic steps.

     Now if your statistic is too low with the multiple scattering, 
instead of shooting one proton you may of course shoot many of them : 
/run/beam 10000, instead of /run/beam 1. Did I missed something ?

Cheers,
Marc


Le 09/12/2010 12:30, Maciej Sznajder a écrit :
> *** Discussion title: Event and Track Management
>
> Hello Forum:)
>
> I have got a problem with very simple physical experiment. I shoot one
> proton to the detector which is an Aluminium foil with thickness 8 mu m.
> By use of the G4hMultipleScattering (one of the physical process which I
> use in the physics list) I have got a good result, the 100keV proton
> stops in about 1 mu m. I can compare the result with SRIM code. BUT
> number of events for this proton is: 2. So the proton hit the detector
> (first event) and it stops (second event).
>
> I can switch off G4hMultipleScattering and switch on
> G4CoulombScattering. The result is also correct, the proton stops at
> depth approx. 1 mu m. But in that case I have got approx. 15 000
> events!! I also use G4hIonisation and G4hBremsstrahlung (Bremstrahlung
> just in case:p)
>
> NOW, in the SRIM code I have got approx. 25 events for one proton which
> travels through the detector. Is there a possibility to change precision
> of calculations??, because 15 000 events (for G4CoulombScattering) is to
> much and 2 events (for G4hMultipleScattering) is to small. In my problem
> I need more data then 2 events, but not to much, like 15000 events.
>
> Best regards, Sznajder
>
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