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## Re: Discrepancy between definition of materials using effective atomic number and true atomic number

Forum: Medical Applications
Re: Discrepancy between definition of materials using effective atomic number and true atomic number (David Oxley)
Date: 20 Oct, 2010
From: michel maire <michel maire>

 On Fri, 01 Oct 2010 15:38:48 GMT, David Oxley wrote: ```> Dear all, > > I am trying to define materials in G4 using their effective atomic > number. Later I also want to look at effective electron density, but > that will not be the subject of this post. > > This is because I want to know how significantly an error in the > effective atomic number effects the range of protons. So I want to > define the material in two different ways, add an uncertainty to one and > observe the difference in the Bragg peak. > > As a starting point, I have defined water using both means and hoped to > get the same answer, but I dont. ``` ` There is no theoritical foundation to choose Zeff = Zmean.` ``` 1- dE/dx is proportional to electron density. This leads to the definition of (Z/A)eff = w1 Z1/A1 + w2 Z2/A2 Then one can choose Zeff and Aeff according to this ratio``` ` 2- if one choose Aeff = Amol, then one find Zeff = Zmol (=10)` ``` 3- Ionisation cross section is proportional to atom density (not electron). This gives a definition of Aeff : 1/Aeff = w1/A1 + w2/A2 (in case of water one find Aeff = 18/3). Hence Zeff = 10/3 ``` ` I let you play with TestEm0 and TestEm7 (in examples/extended/electromagnetic) to see the effects of these various choices.` ``` Michel ```

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