The Geant4 class category diagram is shown in Figure 3.1

Categories at the bottom of the diagram are used by virtually all higher categories and provide the foundation of the toolkit.

The

category covers the system of units, constants, numerics and random number handling.

The two categories:

implement facilities necessary to describe the physical properties of particles and materials for the simulation of particle-matter interactions.

The

module offers the ability to describe a geometrical structure and propagate particles efficiently through it.

Figure 3.1.  Geant4 class categories

Above these reside categories required to describe the tracking of particles and the physical processes they undergo. The

category contains classes for tracks and steps, used by the

category, which contains implementations of models of physical interactions: electromagnetic interactions of leptons, photons, hadrons and ions, and hadronic interactions.

All processes are invoked by the

category, which manages their contribution to the evolution of a track's state and provides information in sensitive volumes for hits and digitization.

Above these the

category manages events in terms of their tracks and the

category manages collections of events that share a common beam and detector implementation. A

category allows the handling of pile-up.

Finally capabilities that use all of these categories and connect to facilities outside the toolkit through abstract interfaces, provide visualization, persistency and user interface capabilities.