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Forum: Event and Track Management
Re: Question Forced Bremsstrahlung only once (Dipanwita Banerjee)
Date: 30 Jan, 2015
From: Marc Verderi <Marc Verderi>

Dear Dipanwita,

Please have a look at the biasing examples examples/extended/GB01 -to 
change the cross-section- or GB02 -to force the interaction inside the 
volume-.

GB01 is close to what you describe. In GB01, you will edit the 
src/GB01BOptrChangeCrossSection.cc file and change the value of 
XStransformation variable to scale the cross-section and, editing 
src/GB01BOptrMultiParticleChangeCrossSection.cc , you can modify the 
number of times you accept the biased cross-section is responsible for 
an interaction, look for "if ( fnInteractions > 4 )        return 0;" ; 
then look at exampleB01.cc to select for which particles you will 
activate the biasing.

GB02 would be interesting as it forces the interaction, but it follows a 
scheme close to the one of MCNP, which is making too many things 
compared to what you need. But I can help you producing something 
simpler, just forcing the brem process, if you are interested in.

Please note that these biasing options are rather new to Geant4 
(appeared in 10.0, with recommended version 10.1). At this times they 
have been essentially tested with neutral particles, but a thin target 
as you have should be fine with charged particles.

Cheers,
Marc



On 01/30/2015 12:47 PM, Dipanwita Banerjee wrote:
> *** Discussion title: Event and Track Management
>
> Dear Sir,
>           I would like to estimate my background of Bremsstrahlung interactions of a 20 GeV electron beam in a 5micron Tungsten target. As the cross-section is quite low I would like to force the Bremsstrahlung interactions in the volume every event to improve my statistics. However I want the interaction to occur only once and not more. I have tried overwriting the mean free path and the cross-section. In either case as its probabilistic putting the mean free path to my target thickness doesn't help as the interaction can occur before or after the mfp distance and this does not ensure one interaction always. If I increase the cross-section there are too many interactions per event. If I change the cross-section such that the mfp would come to the target thickness again I may have more than one or no interaction at all in some instance.
>
> I checked that Geant4 calculates the interaction points before the start
> of the run, so i can't put a flag that if one interaction occurs the
> cross-section should go to 0, as they are pre-calculated.
>
> Is there a way to achieve what I am trying to do. That is one
> bremsstrahlung interaction each event in a 5 micron target.
>
> Thanks, Dipanwita
>
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