Combining two major ATLAS inner detector components
The semiconductor tracker is inserted into the transition radiation tracker for the ATLAS experiment at the LHC. These make up two of the three major components of the inner detector. They will work together to measure the trajectories produced in the proton-proton collisions at the centre of the detector when the LHC is switched on in 2008. (Image: CERN)

The Inner Detector

It is the first part of ATLAS to see the decay products of the collisions

It is very compact and highly sensitive. It consists of three different systems of sensors all immersed in a magnetic field parallel to the beam axis. The Inner Detector measures the direction, momentum, and charge of electrically-charged particles produced in each proton-proton collision.

The main components of the Inner Detector are: Pixel Detector, Semiconductor Tracker (SCT), and Transition Radiation Tracker (TRT).

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Pixel Detector

  • 92 million pixels (92 million electronic channels).
  • Silicon area approx. 1.9m2. 15 kW power consumption
  • Pixel size 50 x 400μm2 for the external layers and 50 x 250 μm2 for the innermost layer (IBL)
  • 4-barrel layers with 1736 sensor modules
  • 3 disks in each end-cap with 288 modules
Semiconductor Tracker

Semiconductor Tracker

  • A silicon microstrip tracker consisting of 4,088 two-sided modules and over 6 million implanted readout strips (6 million channels)
  • 60m2 of silicon distributed over 4 cylindrical barrel layers and 18 planar endcap discs
  • Readout strips every 80μm on the silicon, allowing the positions of charged particles to be recorded to an accuracy of 17μm per layer (in the direction transverse to the strips)
Transition Radiation Tracker

Transition Radiation Tracker

  • 350,000 read-out channels
  • Volume 12m3
  • Basic detector element: straw tube with 4mm diameter, in the centre a 0.03mm diameter gold-plated tungsten wire
  • 50,000 straws in Barrel, each straw 144 cm long. The ends of a straw are read out separately
  • 250,000 straws in both endcaps, each straw 39 cm long
  • Precision measurement of 0.17 mm (particle track to wire)
  • Provides additional information on the particle type that flew through the detector, i.e. if it is an electron or pion


The first ATLAS inner detector end-cap after complete insertion within the Liquid Argon Cryostat.
The first ATLAS inner detector end-cap after complete insertion within the Liquid Argon Cryostat.
The installation of the ATLAS inner detector end-cap C.
The installation of the ATLAS inner detector end-cap C.
An ATLAS inner detector end-cap is placed in its cryostat. The instrumentation housed inside the inner end-cap must be kept cool to avoid thermal noise. This cooling is achieved on ATLAS by placing the end-cap inside a liquid argon cryostat. The end-cap measures particles that are produced close to the direction of the beam pipe and would otherwise be missed.

This colorful 3D animation is an excerpt from the film "ATLAS-Episode II, The Particles Strike Back." Shot with a bug's eye view of the inside of the detector. The viewer is taken on a tour of the inner workings of the transitional radiation tracker within the ATLAS detector. Subjects covered include what the tracker is used to measure, its structure, what happens when particles pass through the tracker, how it distinguishes between different types of particles within it. (Video: CERN)