|Message: Re: finding secondary neutrons||Not Logged In (login)|
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This is considerably more complicated than a single "command".
For computations like this, G4beamline can usually provide an answer much more quickly than writing your own Geant4 program. It reads an ASCII file defining the simulation, and writes a Root file containing output NTuples (traces of tracks, values of tracks that hit each virtualdetector, etc.). The learning curve for G4beamline is considerably lower than for Geant4 itself; this is a trade-off between convenience and generality.
For this computation, G4beamline would require about 10 lines of input to define the physics list to use, the input beam, the geometry of the system, and the location and size of a virtualdetector representing the "planar target". 10,000 beam protons would run in a minute or less, and Root could be used to generate histograms of all particles that hit the virtualdetector, giving the result you need.
http://g4beamline.muonsinc.com -- It is free and open source. It includes a program called historoot that does for Root histograms what G4beamline does for Geant4 -- greatly simplifies the user interface.