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Forum: Biasing and Scoring
Re: Question Transmitted Intensity Data from Lead not building correct Linear Attenuation Coefficient (Sahra)
Date: 01 Feb, 2017
From: michel maire <michel maire>

On Tue, 31 Jan 2017 15:43:18 GMT, Sahra wrote:
> Hi, i would appreciate any help i can get with this, thank you for
> reading,
> 
> I have built a simulation of a very thin lead cylinder inside a vacuum.
> I placed the envelope with the particle gun, behind the lead cylinder,
> and the idea is to 'count' the number of gamma rays transmitted through
> this lead.
> 
> I vary the thickness of the lead, each time, and build a graph of
> thickness (cm) against Ix/Io(Transmitted Flux/Initial Flux). However,
> when calculating the function of this curve, the measurements i
> collected using geant4 do not match the NIST Tables of Linear
> Attenuation Coefficients listed on the web.
> 
> I simulate 10,000 gamma rays at 6MeV, and what i should get is a linear
> Attenuation Coefficient of ~0.505. Instead what i get is -0.022 after
> three simulations:
> 
> 6MeV     x  Incident Intensity  Transmitted Intensity   
> Pb. Thickness (cm)      I(o)    I(x)    Ix/Io
>       0.22             10000    9943   0.9943
>       0.3125           10000    9908   0.9908
>       0.625            10000    9849   0.9849
> 
> I am using a very simple detector construction, Primary Generator Action
> and Action Initialization.
> 
> My first thought was maybe the physics list i was using, which is
> FTFP_BERT had a significant play on this, but i do not understand
> physics lists enough to be able to make a judgement.
> 
> Even a link to a somewhat similar question on this forum would be much
> appreciated.
> 
> Thank you so much for your help, Sahra
> 

  You may run example TestEm13 with the attached macro.
  You will see that the cross sections are in agreement with nist-xcom values :

 Photon    Incoher. Photoel. Nuclear  Tot. wo/ 
 Energy    Scatter. Absorb.  Pr. Prd. Coherent 

 6.000E+00 1.749E-02 9.894E-04 2.523E-02 4.382E-02 

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/02/01/13.59-71254-sahra.mac.txt

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