|Message: Re: Coherent Diffusion of gamma in physics list||Not Logged In (login)|
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User Alain FERRAGUT wrote:
>> Hi, Let's take the case of lead for gamma energy of about 100 keV. For >> this example, I will take the "Compilation of X-Ray Cross section" from >> Lawrence radiation Laboratory (Livermore). These are old data from 1969 >> but still good enough for our discussion. What do we see in this table ? >> >> At 100keV : >> --> PhotoCrossSection = 1777 barns/atom >> --> CoherentCrossSection (Rayleigh) = 67.27 barns/atom >> --> IncoherentCrossSection (Compton) = 34.34 barns/atom >> >> It is true that, compared to photoelectric effect, coherent diffusion of >> gamma is very small. But, on the other hand, Compton diffusion is taking >> into account by the physics list (LHEP_PRECO_HP), although the cross >> section of Compton process is very small compared to photo effect. It is >> even smaller than coherent scattering. So, it is logical to include >> Rayleigh process if Compton process is included, even far above 1keV >> gamma energy.
I admit that one would include Rayleigh to be coherent.
The standard em package, inherited from Geant3, is designed to provide an as efficient as possible shower development in the GeV->TeV region. Such showers are not very sensitive to the details of low energy gamma
processes: including Rayleigh would degrade the efficiency for praticallynothing. However, it is our plane to include in 'standard' an implementation of Rayleigh which would provide a good compromise between accuracy and efficiency. You can include the lowEnergyRayleigh, which need the dataset G4EMLOW
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