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 tracker 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) consists of six concentric layers. Finally two endcaps (TEC) close off the tracker. Each has silicon modules designed differently for its place within the detector.
This part of the tracker contains 15,200 highly sensitive modules with a total of 10 million detector strips read by 80,000 microelectronic chips. Each module consists of three elements: a set of sensors, its mechanical support structure and readout electronics.
Silicon sensors are highly suited to receive many particles in a small space due to their fast response and good spatial resolution. The silicon detectors work in much the same way as the pixels: as a charged particle crosses the material it knocks electron from atoms and within the applied electric field these move giving a very small pulse of current lasting a few nanoseconds. This small amount of charge is then amplified by APV25 chips, giving us “hits” when a particle passes, allowing us to reconstruct its path.
Due to the nature of their job, the tracker and its electronics are pummeled 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.
The charge on each microstrip is read out and amplified by an Analogue Pipeline Voltage (APV25) chip. 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. The APV25 stores the signals in a memory for several microseconds and then processes them before sending to a laser to be converted into infrared pulses. These are then transmitted over a 100m fibre optic cable for analysis in a radiation-free environment. The tracker uses 40,000 such fibre optic links providing a low power, lightweight way of transporting the signal. Much of the technology behind the tracker electronics came from innovation in collaboration with industry.