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Idea Re: Physics for Beamdump-simulations 

Forum: Physics List
Re: Question Physics for Beamdump-simulations
Date: 15 Oct, 2010
From: Zach Hartwig <Zach Hartwig>

Hi Dirk,

> we try to to simulate the background coming from some beamdump to our
> detector. We have 500MeV positrons incident to the beamdump. From the
> discussions, I would conclude that the physics list QGSP_BERT_HP is a
> good start, although we are in the "low-energy regime". Is this right?

I've done some simulation work of a beam dump for 250 MeV protons. In that case, the QGSP_BIC model (the Geant4 binary ion cascade) tends to work a bit better at energies less than 1 GeV for the spallation reactions. This comes from my own validation tests as well as recommendations from the G4 hadronic developers. If you expect intermediate energy (<1 GeV) cascades of hadronic particles, the "BIC" model is the way to go. If you require the accurate transport of neutrons, you should certainly use the "HP" model, which provides data-driven transport of neutrons below 20 MeV and parameterized neutron transport models above 20 MeV. If that's the case, QGSP_BIC_HP is probably the best default physics list to use. Both lists should be capable of handling your EM requirements, although you should make sure. See Below.

> But to understand in more detail what happens, I would like to
> understand how this physics list is made up (from a programmers
> viewpoint) and what is included (from a physicists viewpoint). In
> addition I would like to switch on and off different contributions, for
> example neturons emitted from the dump.

Descriptions of the physics lists and recommendations for use can be found in several places:

In addition, I'd recommend looking at the source for the "physics list builders" that create the physics lists. This allows you to see exactly what physics is being built into the QGSP_BIC and QGSP_BIC_HP lists mentioned above.

You can find the relevant lists in: $G4INSTALL/source/physics_lists/lists/include

The "builders" that are primarily used to add physics to these lists are in: $G4INSTALL/source/physics_lists/builders/

Poke around in those directory and you should an excellent idea of what physics models are included in the lists.

> Is there any documentation available which could help to understand the
> structure of the source-code and how to make up such lists and switch
> on/off or to exchange certain processes?

Check the hadronic examples at: $G4INSTALL/examples/extended/hadronic

Both examples create custom physics lists from the "builders" above and demonstrate how you can switch lists and physics at run-time. Additionally, processes can be deactivated/activated at runtime with the command:

 /process/inactivate <ProcessName>" 


 /process/activate <ProcessName".  

You can see a list of registered physics processes with:


> In addition I get Warnings like
> NeutronHP: /Capture/ file for Z = 20, A = 48 is not found and NeutronHP
> will use
> /home/kram/geant4/data/G4NDL3.13/Capture///CrossSection/20_nat_Calciu

These are just warnings. They indicate that the data file for neutron cross sections used in transport for a particular isotope is not available in the G4NDL library. Typically, this is because the data file does not exist in the ENDF libraries that are used to create G4NDL. This is out of Geant4's control. In this case, Geant4 will default to the next best thing, ie, using the cross section for the natural elemental abundance.

Hope that helps.


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