GEAR - Telescope Geometry Description


On the software side the different telescope geometries are implemented using the Geometry API for Reconstruction (GEAR) markup language. GEAR is a geometry description toolkit which is part of the ILC reconstruction software. It implements an abstract interface for the layout description of detectors for event reconstruction. The geometry described in a GEAR file is simplified and therefore cannot be used for a detailed Monte Carlo simulation of the detector. For reconstruction on the other hand only a few parameters are enough since no precise material allocation information is needed to calculate the particle behavior.

The integration into the EUTelescope framework allows the usage of the unchanged processor chain with the same parameters for different detector geometries, switching between them just by loading another GEAR file. The processors within the chain obtain all information needed for their algorithm directly from the GEAR framework, e.g. the pixel pitches and plane distances for the coordinate transformation from local on-plane coordinates to the global telescope frame of reference.

GEAR uses XML markups for the detector description, whereof several packages for different purposes are provided. In case of silicon pixel detectors the GearType SiPlanesParameters can be used providing all properties needed to describe a tracking telescope. The XML markup contains a hierarchical description of the detector parts starting with the whole detector as well as global parameters such as magnetic field. The telescope can be divided into layers. Every layer represents one particular telescope plane in the GEAR description. This allows e.g. the usage of several sensor types with different properties such as pixel pitch or sensor thickness in different telescope planes which is especially interesting for telescopes with exchangeable DUT. A layer consists of a ladder and a sensitive part. These elements hold the basic parameters such as position, rotation, size, pitch, and the radiation length of the sensitive detector material. Rotations of telescope planes can be obtained either by using a simple 2 × 2 rotation matrix for the xy plane or the Euler angles for 3-dimensional rotations.IDs.