Electro-nuclear Interactions

Process and Cross Section

Electro-nuclear reactions in Geant4 are handled by the classes G4ElectronNuclearProcess and G4PositronNuclearProcess. The default cross section class for both these processes is G4ElectroNuclearCrossSection which was described in detail in an earlier chapter.

Final State Generation

Final state generation proceeds in two steps. In the first step the electromagnetic vertex of the electron/positron-nucleus reaction is calculated. Here the virtual photon spectrum is generated by sampling parameterized \(Q^2\) and \(\nu\) distributions. The equivalent photon method is used to get a real photon from this distribution.

In the second step, the real photon is interacted with the target nucleus at the hadronic vertex, assuming the photon can be treated as a hadron. Photons with energies below 10 GeV can be interacted directly with nucleons in the target nucleus using the measured \((\gamma, p)\) partial cross sections to decide the final state multiplicity and particle types. This is currently done by the Bertini-style cascade (G4CascadeInterface). Photons with energies above 10 GeV are converted to \(\pi^0\)s and then allowed to interact with nucleons using the FTFP model. In this model the hadrons are treated as QCD strings which collide with nucleons in the nucleus, forming more strings which later hadronize to produce secondaries. In this particular model the remnant nucleus is de-excited using the Geant4 precompound and de-excitation sub-models.

This two-step process is implemented in the G4ElectroVDNuclearModel. An alternative model is the CHIPS-based G4ElectroNuclearReaction [DKW00c]. This model also uses the equivalent photon approximation in which the incoming electron or positron generates a virtual photon at the electromagnetic vertex, and the virtual photon is converted to a real photon before it interacts with the nucleus. The real photon interacts with the hadrons in the target using the CHIPS model in which quasmons (generalized excited hadrons) are produced and then decay into final state hadrons. Electrons and positrons of all energies can be handled by this single model.