\chapter{Materials}

\section{Design Philosophy}

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. 

\section{Class Design}

The object-oriented design of the 'materials' related classes
is shown in the class diagram: 
Fig. \ref{figure:materials-1}. 
The diagram is described in the Booch notation. 


\begin{figure}[ht!]
\includegraphics[angle=0,scale=0.7]{OOAnalysisDesign/Materials/classDgmMaterials.eps}
\vspace{10pt}
\caption{Materials}
\label{figure:materials-1}
\end{figure}


\section{Status of this chapter}

27.06.05 section on design philosophy add (from Geant4 general paper) by 
         D.H. Wright \\



