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Forum: Fast Simulation, Transportation & Others
Re: Question Why did my Method not Work (Sahra)
Date: 10 Feb, 2017
From: michel maire <michel maire>

On Fri, 10 Feb 2017 10:10:38 GMT, Sahra wrote:
> Hi, this is mostly a question of why my particular technique in finding
> area cross-sections and attenuation coefficients of materials in Geant4
> did not work.
> 
> I currently understand that Extended Example 'TestEm13' is the main
> example provided by Geant4 to enable you to find the
> cross-section/attenuation coefficients of different materials, which was
> my original project, and now understand how to use that.
> 
> However, before i came across this example, i had come up with a
> particular method of finding the initial and transmitted gamma photon
> intensities through lead, by placing a slice of lead in a envelope, a
> particle gun on the edge of the envelope so that it shoots perpendicular
> to the slice of lead, and a box_mesh/scorer on the other side of the
> lead.
> 
> I set my own initial intensity of gamma photons, say 10,000, and the
> scorer/box_mesh which i implemented via commands, would 'count' or
> 'score the gammas that made it through the lead, onto the
> scorer/box_mesh.
> 
> The scorer would only count gamma photons making it on to the scorer
> via. 'FlatSurfaceCurrent' which is a method to count particles injecting
> the surface of the scorer without taking into account the angle at which
> the photon hits it.
> 
> I get good values in an external text file i created like, 9064/10,000
> particles were counted, but when i start building up data of thickness
> of lead (which i change manually in between events and then re-build the
> application again) against the decreasing number of photons detected, i
> get an attenuation coefficient value that is not equal to the actual
> value I am meant to get.
> 
> What have i done wrong or understood wrong? - My envelope and world
> volumes are both as close to vacuum as you can get. - My detector is the
> Geant4 default lead material of varying thickness. - My scorer is a
> simple box mesh, divided into 50 by 50 bins i.e. (50,50,0).
> 
> I really wish to understand how to use Geant4 to a really good level, so
> I would appreciate any and all feedback on this, thank you for helping.
> 
> Sahra
> 

 1- To compute cross sections from transmitted particles, you must count the unaltered beam, e.g. the incident particles which have not interact.
  Incident gammas which suffer a Compton interaction are scattered and can be transmitted. They must not be counted.
  To better understand what I mean, I invite you to run TestEm5 interactively with the attached macro.

 2- Another method to evaluate cross sections is illustrated TestEm14. From the computing point of view, this approach is much more efficient.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/02/10/07.27-27209-sahra.mac.txt

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