Solar Orbiter's Energetic Particle Detector (EPD) HET
The High-Energy Telescope (HET) will measure electrons, protons, and heavy ions. Electrons are covered across the energy range from 300 keV up to about 20 MeV, protons from 10 – 100 MeV, and heavy ions from ~20 to 200 MeV/nuc (species dependent, see next Figures). HET allows separation of the helium isotopes down to a 3He/4He isotope ratio of about 1%. Thus, HET covers the energy range which is of specific interest for studies of the space environment (space weather) and will perform the measurements needed to understand the origin of high-energy events at the Sun which occasionally accelerate particles to such high energies that they can penetrate the Earth’s atmosphere and be measured at ground level (ground-level events, GLEs).
These measurement capabilities are reached by a combination of solid-state detectors
and a scintillator calorimeter which allows use of the dE/dx vs. total E technique for
particle identification and energy measurement. The upper limits on energy listed above
refer to particles (ions) stopping in the scintillator, careful modelling of HET properties
will allow discrimination of forward/backward penetrating particles.
HET consists of two sensor double-ended heads, one pointing sun/anti-sunward, the
other out of the ecliptic. Thus, HET has a total of four viewing directions. Both HET
sensors are identical and consist of a double-ended set of solid state detectors and a
high-density calorimeter scintillator. Current trades still involve BGO and GSO as
scintillator materials.
The HET entrance collimator is protected by a 50 microns thick aluminium foil that
reduces the low-energy particle flux on the front detector. In addition, the front detector
is divided into concentric segments that allow us to reduce the proton count rate during
high intensity events. In such situations, the thresholds on the larger segment are
increased to beyond the energy deposit of protons. This scheme retains the detection
power for the much rarer heavy ions while reducing the counting rate for the abundant
protons. HET will trigger on the second detector in the telescopes.
Figure: Simple view of the HET assembly
