Why ECAL Matters
Positrons, with the same charge as protons but vastly different mass, pose a unique challenge for particle detection. Due to their similar rigidity at high energies, positrons cannot be distinguished from low-energy protons by magnetic fields alone. With positrons occurring at a rarity of 1 per 100,000 protons, and antiprotons similarly scarce, efficient separation methods are imperative. Enter the Electromagnetic Calorimeter (ECAL), a specialized detector boasting an impressive identification power of one positron per 100,000 protons. This precision is further augmented by the Transition Radiation Detector (TRD).
Additionally, ECAL serves as a direct measure of high-energy photons (γ), accurately determining both their energy and direction.
Understanding ECAL Layers
When a high-energy electron (e+), positron (e–), or photon (γ) traverses a high atomic number (Z) material like lead, it initiates an electromagnetic shower. This cascade of secondary particles, resulting from bremsstrahlung and pair production, distinguishes electromagnetic showers from the broader hadronic showers produced by incident protons. ECAL meticulously reconstructs these showers in 3D across 18 different depths, precisely delineating their longitudinal and transverse profiles.
Constructing the ECAL
Comprising nine super-layers, ECAL forms a pancake structure with an active area of 648×648 mm² and a thickness of 166.5 mm. Each super-layer, 18.5 mm thick, consists of lead foils interleaved with scintillating fibers, meticulously arranged to facilitate imaging capabilities. This design, with fibers oriented alternately along the x- and y-axes, ensures accurate particle tracking and energy measurement. With an average density of 6.9 g/cm³ and a weight of 496 kg, ECAL achieves exceptional precision while remaining compact.
In-Depth: AMS γ-ray Measurement
AMS employs two complementary methods to measure high-energy γ-rays:
- Single Photon Mode: ECAL directly detects γ-rays, functioning independently to trigger and record gamma ray events with exceptional precision, especially up to energies exceeding 300 GeV.
- Conversion Mode: When a high-energy photon converts into an electron/positron pair within or before the Tracker, the resultant pair is produced in the forward direction and separated by the magnetic field. By measuring the curvature of each particle, AMS reconstructs the photon’s energy and arrival direction. The orientation information from the Star Tracker further refines the direction determination with respect to fixed stars.
In summary, ECAL’s meticulous design and functionality empower AMS with unparalleled precision, advancing our understanding of high-energy particles and cosmic phenomena.