The ECAL

Motivation

ECAL-300x122.jpgThe positron has the same charge and sign as a proton, but only 1/2000th of the mass. A high-energy positron can have the same rigidity as a low-energy proton, hence they cannot be seperated by the magnetic field. Additionally, the positron is a very rare component of cosmic rays, with 1 positron for every 10.000 protons. The same argument can be made for negative-sign particles e.g. the electron and antiproton (with a ration of 100:1). Therefor, an efficient way of seperating theses particles is needed.

The electromagnetic calorimeter (ECAL) of AMS-02 in a specialized detector with precisely this purpose. It can distiguish between positrons and protons which an identification power of 1 positron over 100.000 protons. The positron/proton rejection power is further increased with the help of another detector: the TRD.

The ECAL can also be used to measure direct high-energy photons with an accurate determination of energy and direction.

Background

ECAL_layers-300x237.jpgWhen a high-energy positron, electron or photon passes through matter with a high Z (e.g. lead), many more electrons, positrons and photons are produce, albeit with a lower energy. This production of secondary particles is called a electromagnetic shower, and is the result of the combination of two effects: the bremsstrahlung (german for braking radiation), the production of photons by positrons and electrons, and the pair production which is the conversion of a photon into electron-positron pairs. The shower ends when the secondary particles are absorbed by or leave the material.

In contrast, an incident proton in a different way, producing a hadronic shower, which has a different shape. It consists of the production of a variety of secondary particles (pions, kaons, etc.) resulting in a wider shower.

The AMS-02 ECAL is able to present a 3D shower profile at 18 different depths. This profile allows an accurate destinction between positrons and protons. Additionally, electons and positrons with an energy of below 1 TeV produce showers almost completely contained in the ECAL. This allows for the reconstruction of the particle's energy which is then proportional to the sum of the ECAL signals.

The shower shape can also be used to reconstruct the direction of the incident particle within a few degrees. This feature is important for the measurement of high-energy photons.

Construction

The ECAL ist built like a pancake stack, consisting of 9 super-layers which results in an active area of 648x648 mm2 and a thickness of 166.5 mm. Each super-layer is 18.5 mm thick and is made of 11 grooved, 1 mm thick lead foils interleaved with layers of 1 mm thick scintillating fibers glued together with an epoxy resin. By stacking theses super-layers alternatingly parallel to the x-axis (4 layers) and the y-axis (5 layers) the two-dimensional imaging capability is achieved. The stack has an average density of 6.9 g/cm3, resulting in a total weight of 496 kg.