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

Located just 3.3 cm from the LHC beam line, the Pixel Detector is the first point of detection in the ATLAS experiment. It is made up of four layers of silicon pixels, with each pixel smaller than a grain of sand. As charged particles burst out from the collision point, they leave behind small energy deposits in the Pixel Detector. These signals are measured with a precision of almost 10 μm to determine the origin and momentum of the particle. The Pixel Detector is incredibly compact, with over 92 million pixels and almost 2000 detector elements.

  • 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

The Semiconductor Tracker surrounds the Pixel Detector and is used to detect and reconstruct the tracks of charged particles produced during collisions. It consists of over 4,000 modules of 6 million “micro-strips” of silicon sensors. Its layout is optimised such that each particle crosses at least four layers of silicon. This allows scientists to measure particle tracks with a precision of up to 25 μm - that’s less than half the width of a human hair!

  • 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
Semiconductor Tracker

Transition Radiation Tracker

The third and final layer of the Inner Detector is the Transition Radiation Tracker (TRT). Unlike its neighbouring sub-detectors, the TRT is made up of 300,000 thin-walled drift tubes (or “straws”). Each straw is just 4 mm in diameter, with a 30 μm gold-plated tungsten wire in its centre. The straws are filled with a gas mixture. As charged particles cross through the straws, they ionise the gas to create a detectable electric signal. This is used to reconstruct their tracks and, owing to the so-called transition radiation, provides information on the particle type that flew through the detector, i.e. if it is an electron or pion.

  • 350,000 read-out channels
  • Volume 12m3
  • Straw tubes with 4mm diameter, with centred 0.03mm diameter gold-plated tungsten wire
  • 50,000 straws in Barrel, each straw 144 cm long.
  • 250,000 straws in both endcaps, each straw 39 cm long
  • Precision measurement of 0.17 mm (particle track to wire)
Transition Radiation Tracker


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)