Silicon Strip Detectors in the ATLAS Experiment

Mechanism of detection

Particle physics needs detectors which can determine the position of particles with an accuracy of 0.01 mm, have minimal thickness (0.3mm), and have very fast ( 0.000000025 second) time response.

Silicon, a semiconductor, can be fabricated in two forms; n type, with a surplus of electron sites in the crystal lattice, and p type, with a deficit of electron sites in the crystal lattice.

The majority of silicon detectors consist of n type bulk material. The back face has an aluminium contact over the complete surface. The front face has p type silicon strips implanted in the surface. These p type strips aluminium strips on their surface. The aluminium strips are separated from their associated p type silicon strips by a thin insulator. An electric field is applied between the p strips and the back face.

When a charged particle passes through a silicon detector it creates ionisation in the bulk of the silicon. This frees electrons from the atoms of the silicon and leaving these atoms with an electron vacancy. These vacancies are referred to as "holes".

The "holes" "drift" in the electric field towards the negatively charged p type strips. The electrons "drift" towards the positively charged back plane.

When the "holes" reach the p type strip they are collected and induce a measurable charge on the associated aluminium strip. The aluminium strips are connected to sensitive electronic read out channels.

By recording which electronic channel fired, it is possible to determine where the charged particle passed through the detector.

The ATLAS Inner Tracker Detector

The Inner Tracker is at the heart of the ATLAS detector. It consists of 3 different types of detector including silicon strip detectors in the Barrel and Forward SCT (Semi-Conductor Tracker).

  • Transition Radiation Tracker : This consists of long thin tubes 40-70cm long and 0.5cm in diameter, filed with gas and with a single wire running along their length. This detector provides about 30 measurements along a track with a resolution of about 0.2mm.
  • Semi-Conductor Tracker : Inside the TRT, this detector is constructed from Silicon wafers 6cm square. Particles are detected by strips 0.075mm apart running along the detector. The path of the particle is measured with a point precision of ~0.02mm perpendicular to the strips.
  • Pixel Detector : The innermost part of the ATLAS detector, this is also made from Silicon wafers, but in these detectors the sensitive element is a "Pixel" 0.05mm by 0.3mm.