Design of the High Energy Particle instrument for electrons (HEPe)for ERG mission

Abstract: It is known that the Earth’s inner magnetosphere is the region where plasma
particles have a wide range of energy from a few eV to 10 MeV. Though
abundant in situ satellite measurements have been made in the past decades,
the Earth’s inner magnetosphere is still an unclear region in terms of
scientific investigation, particularly in the area of space plasma dynamics.
However, this region is very important as a natural laboratory where
high-energy particle acceleration can be directly measured in the dipolar
field configuration, and also for human activities in space including space
weather prediction as well. Therefore, a Japanese exploration mission
termed
Energization and Radiation in Geospace (ERG) was decided to be launched in
the coming solar maximum around 2014 to observe the energetic particles with
a wide range of energy, electromagnetic field and plasma waves
simultaneously.
Measurements of the high energy electrons are the key in this mission. A
High Energy Particle instrument for electrons (HEPe) will be developed with
Solid State Detector (SSD). HEPe has three assemblies with a field of view
(FOV) of more than 60° x 10° each, and therefore can fulfill a 4π
steradian coverage after one full spin period. The Single-sided Strip
Silicon Detector (SSSD) is used so as to give the angular information of
the particle with an angular resolution 10° x 10°. In order to cover the
energy range from 50 keV to 2 MeV, two sensor heads with different geometry
factors but similar structures are designed for each assembly, called HEPe-
Low (50 keV – 1 MeV) and HEPe-High (700 keV – 2 MeV) respectively. A
12μm aluminum film plus a thin layer of SSSD is used in HEPe-Low for
the discrimination of ion's contamination. A 250μm aluminum foil is
used in HEPe-High for the elimination of the lower energy electrons. All
the sensor heads are integrated in a cylinder shape and the total size is
less than 200mm in diameter.