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Forum: Hadronic Processes
Re: Question Thermal neutron scattering on plexiglass (Matthias Volnhals)
Date: 06 Sep, 2012
From: Edward Leggett <Edward Leggett>

Hi,

I have been messing around with G4NeutronHPThermalScattering, and I can't tell you what is the best model for your situation, but I can at least tell you how to turn on G4NeutronHPThermalScattering so that you can see the difference yourself.

> As far as I
> know, this is not enabled by default in Geant4.9.3.p2, which is the
> version I am using. 

No physics processes are enabled by default in any version of Geant4, it always depends on your physics list. However, none of the reference physics lists that I have found include thermal scattering. You will need to create your own physics list and register G4NeutronHPThermalScattering. The easiest way to do this is to create a custom G4VPhysicsConstructor and use it in your own modular physics list. I simply modified G4HadronElasticPhysicsHP to include thermal scattering below 4 eV (note: elastic hadronic interactions are always in a separate module from the rest of the hadronic interactions). Attached are the modified files (ThermalNeutronScattering.hh/.cc). You should be able to just add these to your source and then add the following line to your modular physics list: this->RegisterPhysics( new ThermalNeutronScattering(verboseLevel) );

> Would "G4NeutronHPThermalScattering" actually
> apply to G4_PLEXIGALSS? 

No, the list of materials applicable to G4NeutronHPThermalScattering can be found in G4NeutronHPThermalScatteringNames.cc. The NIST materials are listed at bottom, i.e. G4_something. The group at the top are elements, i.e. if you define an element with this exact name and then use it in a material, G4NeutronHPThermalScattering will be applied. A run time message will list all materials/elements that are recognized by G4NeutronHPThermalScattering.

Now, even though "PLEXIGLASS" is not one the relevant materials, you might try using polyethylene as an approximation. If you use G4_PLEXIGLASS, then G4NeutronHPElastic will be used, which contains no S(&#945;,&#946;) information and is based on a free gas approximation. What model is best is a question for a neutronics expert. I did notice that T. Koi uses polyethlyene as an approximation for PMMA in a presentation that I found on the web.

Cheers,

Jed Leggett

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2012/09/06/10.16-21769-lNeutronScattering.cc.txt
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2012/09/06/10.16-41759-lNeutronScattering.hh.txt

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