The Electromagnetic CALorimeter (ECAL) is a heavy-lead brick equipped with instrumentation. Incident particles interact in such a dense material producing a shower of low-energy particles. The shape of the shower identifies the particle kind (proton or positron) and the particle total energy.
Why do we need the ECAL?
The positron has the same proton charge and sign, but a 1/2000 mass. Since a high-energy positron could have the same rigidity of a low-energy proton, they cannot be separated by a magnetic field. Positrons are very rare components of cosmic-rays. They have an abundance of 1 every 100,000 protons. We need an efficient way of separating positrons from protons. A similar argument is valid for the negative sign particles, as electrons and antiprotons. Indeed the antiprotons natural abundance is 1 every 100 electrons. ECAL is a specialized detector able to distinguish among positrons/protons and electrons/antiprotons with an identification power of one positron over 100,000 protons. The positron/proton rejection power improves by means of another AMS specialized detector: the TRD.
ECAL is also able to measure directly high-energy photons (γ) with an accurate energy and direction determination.
How does the ECAL work?
When a high-energy e+, e– or γ passes through a material with a high Z – as Lead – many other e+, e– and γ of lower energy are produced. This particle production is called electromagnetic shower and is caused by the interplay of two phenomena: the bremsstrhalung (the german word for braking radiation) or production of photons by positrons and electrons, and the pair production that consists in the conversion of a photon in e+/e– pair. The shower ends either when secondary particles are absorbed in material or when they are able to escape from the material.
An incident proton interacts in a very different way, producing a hadronic shower, which has a totally different shape. The proton shower is characterized by the production of many types of particles (pions, kaons, …) resulting in a wider shower.
ECAL is able to reconstruct a 3D shower profile at 18 different depths. These measurements will give an accurate description of the longitudinal and transverse shower shape allowing the positron/proton showers distinction. Incident e+, e– or γ with energies below 1 TeV produce an electromagnetic shower almost contained in the ECAL. The sum of ECALS signals belonging to the shower is proportional to the energy of the particle.
From the shower shape is also possible to reconstruct the direction of the incident particle. ECAL can reach angular precision of few degrees. This is very important for high-energy photons measurement.
How is the ECAL built?
The calorimeter consists on a pancake composed from 9 super-layers for an active area of 648×648 mm2 and a thickness of 166.5 mm. Each super-layer is 18.5 mm thick and it is made of 11 grooved, 1 mm thick lead foils interleaved with layers of 1 mm diameter scintillating fibers, glued together with epoxy resin. The detector imaging capability is obtained by stacking super-layers with fibers alternatively parallel to the x-axis (4 layers) and y-axis (5 layers). The pancake has an average density of 6.9 g/cm³ for a total weight of 496 kg.
In Depth: The AMS γ-ray measurement
AMS measures high-energy γ-rays energy and arrival direction in two complementary ways.
1) Single Photon Mode: i.e., by the direct detection of ECAL. In this case, ECAL works like a standalone instrument. It can trigger and record the gamma ray event by itself. The ECAL position and energy resolution are very good up to 300 GeV or more.
2) Conversion Mode: i.e., by the photon pair conversion in the Tracker. When a high-energy photon converts into the electron/positron pair before or in the Tracker, the pair couple is produced in the forward direction and is then separated by the magnetic field. We can reconstruct the photon energy and its arrival direction from the electron/positron vertex production angle and from the two separate curvature measurements.
The photon direction with respect to fixed stars is finally derived using the Star Tracker orientation information.