Peculiar neutron flux spectrum


I am trying to get a flux spectrum with very fine bins I have approximately 48,000 bins between 0-15 MeV. As I have custom bin width as per ICRP, I am doing this by command-based scoring in a Macro, Later I use MATLAB to parse the data. I am getting a very weird spectrum (attached). Here my source energy is only 14.1 MeV But I get flux above that energies, shouldn’t there be a cutoff at 14.1? Is this a bug?

My physics list is below.
G4VModularPhysicsList* phys = new QGSP_BIC_HP(1);
phys->RegisterPhysics(new G4ThermalNeutrons(1));

I also get lot of zeros for many bins.

My Macro is also available at:
Macro File

My output file is available at:
Output File

Also, this run took 21 days to finish.

Please let me know your suggestions and advice on this result.

Thanks very much in advance.



in gamma-nuclear process target nucleus is destroied and extra energy may be delivered to a secondary neutron. Extend your spectra above 15 MeV and you will see high energy tail.

Concerning empty bins: likely particle steps are too large compared to your bin size.