Gamma-nuclear Interactions

Process and Cross Section

Gamma-nuclear reactions in Geant4 are handled by the class G4PhotoNuclearProcess. The default cross section class for this process is G4PhotoNuclearCrossSection, which was described in detail in the previous chapter.

Final State Generation

Final state generation proceeds by two different models, one for incident gamma energies of a few GeV and below, and one for high energies. For high energy gammas, the QGSP model is used. Incident gammas 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.

At lower incident energies, there are two models to choose from. The Bertini-style cascade (G4CascadeInterface interacts the incoming gamma with nucleons using measured partial cross sections to decide the final state multiplicity and particle types. Secondaries produced in this initial interaction are then propagated through the nucleus so that they may react with other nucleons before exiting the nucleus. The remnant nucleus is then de-excited to produce low energy fragments. Details of this model are provided in another chapter in this manual.

An alternate handling of low energy gamma interactions is provided by G4GammaNuclearReaction, which is based upon the Chiral Invariant Phase Space model (CHIPS [DKW00c, DKW00a, DKW00b]). In Geant4 version 9.6 and earlier a separate CHIPS model was provided for gamma nuclear interactions. Here the incoming gamma is absorbed into a nucleon or cluster of nucleons within the target nucleus. This forms an excited bag of partons which later fuse to form final state hadrons. Parton fusion continues until there are none left, at which point the final nuclear evaporation stage is invoked to bring the nucleus to its ground state.

Low Energy Nuclear Data Model

The LEND model (Low Energy Nuclear Data) utilised evaluated data to sample the final states of gamma-nuclear interactions to high precision. This model is particularly suitable for incident gammas below 20 MeV giving higher precision than the default Bertini Cascade model (The Bertini Intranuclear Cascade Model).

Bibliography

DKW00a

P. V. Degtyarenko, M. V. Kossov, and H. -P. Wellisch. Chiral invariant phase space event generator, i. nucleon-antinucleon annihilation at rest. The European Physical Journal A, 8(2):217–222, jul 2000. URL: https://doi.org/10.1007/s100500070108, doi:10.1007/s100500070108.

DKW00b

P.V. Degtyarenko, M.V. Kossov, and H.-P. Wellisch. Chiral invariant phase space event generator, ii. nuclear pion capture at rest. The European Physical Journal A, 9(3):411–420, dec 2000. URL: https://doi.org/10.1007/s100500070025, doi:10.1007/s100500070025.

DKW00c

P.V. Degtyarenko, M.V. Kossov, and H.-P. Wellisch. Chiral invariant phase space event generator, iii: modeling of real and virtual photon interactions with nuclei below pion production threshold. The European Physical Journal A, 9(3):421–424, dec 2000. URL: https://doi.org/10.1007/s100500070026, doi:10.1007/s100500070026.