FAQ:What is a Detector Element?
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From Section 8.1 of the user manual,
'Structure describes a hierarchy of distinct, placed “touchable” volumes (Geant4 nomenclature) also known as Detector Elements (Gaudi nomenclature). Not all volumes are directly referenced in this hiearchy, only those that are considered important.
A Geant4 "touchable" volume is defined as
A "touchable" for a volume serves the purpose of providing a unique identification for a detector element. This can be useful for description of the geometry alternative to the one used by the Geant4 tracking system, such as a Sensitive Detectors based read-out geometry, or a parameterised geometry for fast Monte Carlo. In order to create a "touchable volume", several techniques can be implemented: for example, in Geant4 touchables are implemented as solids associated to a transformation-matrix in the global reference system, or as a hierarchy of physical volumes up to the root of the geometrical tree. A "touchable" is a geometrical entity (volume or solid) which has a unique placement in a detector description. It is represented by an abstract base class which can be implemented in a variety of ways. Each way must provide the capabilities of obtaining the transformation and solid that is described by the "touchable".
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