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Question Re: Forced Bremsstrahlung only once 

Forum: Event and Track Management
Re: Question Forced Bremsstrahlung only once (Dipanwita Banerjee)
Re: None Re: Forced Bremsstrahlung only once (Marc Verderi)
Date: 30 Jan, 2015
From: Dipanwita Banerjee <Dipanwita Banerjee>

Dear Marc,
          Thank you for the suggestion. I tried it and it seems to be working although I get a warning 

G4Exception : BIAS.GEN.14
      issued by : G4InteractionLawPhysical::SetPhysicalCrossSection(..)
Cross-section value passed is negative. It is set to zero !
*** This is just a warning message.

I am not sure why this is. I adapted the code to bias only eBrem processes for electrons in a certain volume and to include at most 1 interaction. For 10 events it seemed to work I am now running more events to see if it changes. Can you tell me why I am getting these warnings and if there is a way to silence them?

Regards, Dipanwita

On Fri, 30 Jan 2015 13:19:07 GMT, Marc Verderi wrote:

> 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
> >
> > -------------------------------------------------------------
> > Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at:
> > http://hypernews.slac.stanford.edu/HyperNews/geant4/get/eventtrackmanage/1239.html
> 

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