The design of the materials category reflects what exists in nature: materials are made of a single element or a mixture of elements, and elements are made of a single isotope or a mixture of isotopes. Because the physical properties of materials can be described in a generic way by quantities which can be specified directly, such as density, or derived from the element composition, only concrete classes are necessary in this category. The material category implements the facilities necessary to describe the physical properties of materials for the simulation of particle-matter interactions. Characteristics like radiation and interaction length, excitation energy loss, coefficients in the Bethe-Bloch formula, shell correction factors, etc., are computed from the element, and if necessary, the isotope composition. The material category also implements facilities to describe surface properties used in the tracking of optical photons. The object-oriented design of the 'materials' related classes is shown in the class diagram: . The diagram is described in the Booch notation.

[Status of this chapter] 27.06.05 section on design philosophy add (from Geant4 general paper) by D.H. Wright Dec. 2006 Conversion from latex to Docbook verson by K. Amako