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<sect2 id="sect.Geom.Intro">
<title>
Introduction
</title>

<para>
The detector definition requires the representation of its
geometrical elements, their materials and electronics properties,
together with visualization attributes and user defined properties.
The geometrical representation of detector elements focuses on the
definition of solid models and their spatial position, as well as
their logical relations to one another, such as in the case of
containment.
</para>

<para>
Geant4 uses the concept of "Logical Volume" to manage the
representation of detector element properties. The concept of
"Physical Volume" is used to manage the representation of the
spatial positioning of detector elements and their logical
relations. The concept of "Solid" is used to manage the
representation of the detector element solid modeling.
Volumes and solids must be dynamically allocated in the user
program; objects allocated are automatically registered in
dedicated stores which also take care to free the memory at
the end of a job.
</para>

<para>
The Geant4 solid modeler is STEP compliant. STEP is the ISO
standard defining the protocol for exchanging geometrical data
between CAD systems. This is achieved by standardizing the
representation of solid models via the EXPRESS object definition
language, which is part of the STEP ISO standard.
</para>

</sect2>
