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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