Neutron interactions
Neutrons interact with matter mainly through the following processes:
- Neutron capture (Geant4 name: nCapture, GEARS process id: 4131)
- Elastic scattering (Geant4 name: hadElastic, GEARS process id: 4111)
- Inelastic scattering (Geant4 name: neutronInelastic, GEARS process id: 4121)
- Neutron induced Fission (Geant4 name: nFission, GEARS process id: 4141)
Gd(n,gamma)
The default HP (high precision) neutron model used in Geant4 does not handle Gd(n,gamma) correctly. This is described in detail in https://indico.cern.ch/event/746466/contributions/3345714/. An alternative model is called photon evaporation, which can be activated by setting up an environment variable:
export G4NEUTRONHP_USE_ONLY_PHOTONEVAPORATION=1
(Note that this will be replaced by a macro command in Geant4 version 11). The dataset used by this model is
export G4LEVELGAMMADATA=/path/to/Geant4/installtion/folder/share/Geant4-??.?.?/data/PhotonEvaporation?.?
Example simulation and analysis codes can be found in https://github.com/jintonic/gears/tree/master/tutorials/physics/neutron/Gd. As for Geant4.10.7, the simulated gamma-ray spectrum is not satisfactory compared to data described in https://academic.oup.com/ptep/article/2020/4/043D02/5819518.
A comprehensive example
In DT.mac, 14 MeV neutrons from a DT generator are shot to a concrete block defined in concrete.tg. Example analysis code:
// check which interaction is dominant
root [] t->Draw("pro","pdg==2112 && pro>2000")
root [] gPad->SetLogy()
// neutrons all end up with being captured (pro==4131)
// draw the last point of a neutron track
root [] t->Draw("x","pdg==2112 && pro==4131")
// draw charges of generated particles
root [] t->Draw("q")
// draw isotopes involved
root [] t->Draw("pdg", "pdg>10000")