GAMMA-400 is a space mission included in the Russian Federal Space Program
and supported by the Russian Federal Space Agency. The main characteristics
of the mission are a high elliptical orbit (initial parameters: perigee 500 km,
apogee 300 000 km), a total mass for the scientific payload of 2600 kg, and a
power budget for the instrument of 2 kW. The experiment is intended to
improve the angular and energy resolutions obtained by other space missions
for gamma-ray and electron detections in the 0.1-3000 GeV energy range. For
100 GeV gamma rays, the expected angular and energy resolutions are ~0.01°
and ~1%, respectively. The apparatus will consist of a finely segmented
converter/tracker (made by thin tungsten layers and sensitive planes of silicon
microstrip detectors), and a deep (≈ 25X0) homogeneous imaging calorimeter
for energy measurement. On the top of the Si-W converter/tracker, a light
multilayer silicon tracking detector will extend the GAMMA-400 measuring
capabilities for low- and medium-energy gamma rays in the range 50-300 MeV.
GAMMA-400 will permit to identify many discrete gamma-ray sources, in
particular at the center of the Galaxy, to study the diffuse gamma-ray
background, and to precisely investigate gamma-ray energy spectrum features
in a wide energy range. The homogeneous and deep calorimeter, besides
providing excellent energy resolution and rejection power, can also be used to
measure cosmic-ray protons and nuclei entering from the sides, thus achieving
a total GF for nuclei exceeding 1 m2sr and enabling the measurement, in a few
years, of the proton flux beyond 1 PeV and the helium flux beyond 0.5
PeV/nucleon.