Geometry in Geant 4

Paper: 348
Session: A (talk)
Speaker: Apostolakis, John, CERN, Geneva
Keywords: simulation, simulation/CAD interfacing, simulation tools

Geometry in Geant 4

John Apostolakis and the RD44 Colaboration.

We describe the handling of key geometrical entities in Geant 4. The
detector geometry is described by means of a hierarchy or set of volumes.
A distinction is made between the positioning of volumes, their
attributes, and their geometrical shape. These concepts correspond to
physical volumes, logical volumes and solids respectively.

Solids can be represented in two different ways: simple shapes as solid
objects (known as Constructed Solid Geometry), and more complex shapes
using their boundary surfaces (boundary representations). The seperation
of the concept of a solid from that of the logical volume, the use of
BREPS and the interface to CAD system geometries using the STEP standard
are key ideas and innovations in Geant 4's geometry.

The concepts of physical and logical volumes and the hierachy built
using them are borrowed from Geant-3. They allow the efficient
representation of complex detectors by reusing subtrees, positioned at
many parts of the detector. An important departure from this arises in the
case of the importation of CAD geometries, which are flat, not

This challenges the module that propagates particle tracks in the
detector's volumes, the navigator. The navigator is responsible for
efficiently finding the intersection of a track with the volumes, when the
path is approximately linear or curved by a magnetic field. In the
context of non-hierachical detector descriptions a new approach for space
subdivision is required and has been invented. This new method of "smart
voxels" extends the concept of "virtual divisions" and provides the power
required to handle flat geometries.