A Multi-Layer Detector

The detector is made up of three main layers, each of which is further subdivided:

1. The tracking detectors are low-density devices. Particles produced in the electron-positron interaction just fly outwards through the tracking detectors, usually losing only a tiny part of their energy. However, if the particles are electrically charged, they cause ionisation of the tracking detector materials as they fly outwards: we detect this ionisation, and by measuring its position we can measure the path that the charged particle followed.
2. The calorimeters are made of high density material, and they are designed to stop most of the particles originating from the electron-positron interaction. When electrons, photons or hadrons stop in the material of the calorimeters, they give rise to showers. We can measure the energy of the incoming particle from the properties of these showers.
3. On the outside of the detector lies the muon system. The muon detectors are gas-filled chambers which detect the passage of charged particles, similarly to the central tracking detectors. Muons are very penetrating particles, and are the only ones which normally pass right through the detector as far as the muon system: so we can tell if a muon was present by looking for signals in these muon detectors.