After the pixels and on their way out of the tracker, particles pass through ten layers of silicon strip detectors, reaching out to a radius of 130 centimetres.
The silicon strip detector consists of four inner barrel (TIB) layers assembled in shells with two inner endcaps (TID), each composed of three small discs. The outer barrel (TOB) – surrounding both the TIB and the TID – consists of six concentric layers. Finally two endcaps (TEC) close off the tracker on either end. Each has silicon modules optimised differently for its place within the detector.
This part of the tracker contains 15,200 highly sensitive modules with a total of about 10 million detector strips read by 72,000 microelectronic chips. Each module consists of three elements: one or two silicon sensors, its mechanical support structure and readout electronics.
The silicon detectors work in much the same way as the pixels: as a charged particle crosses the material it knocks electrons from atoms giving a very small pulse of current lasting a few nanoseconds. This small amount of charge is then amplified by Analogue Pipeline Voltage (APV25) chips, giving us “hits” when a particle passes, allowing us to reconstruct its path. Four or six such chips are housed within a “hybrid”, which also contains electronics to monitor key sensor information, such as temperature, and provide timing information in order to match “hits” with collisions.
Due to the nature of their job, the tracker and its electronics are pummelled by radiation but they are designed to withstand it. To minimise disorder in the silicon this part of the detector is kept at -20oC, to “freeze” any damage and prevent it from perpetuating.
CMS Tracker layers shown in the plane perpendicular to the beam.