Solar Orbiter's Energetic Particle Detector (EPD) - LET
The Low Energy Telescope (LET) is designed to identify elements from H to Ni
with excellent element and energy resolution over the range from ~1.5 to ~60 MeV/nuc
where SEP spectral breaks typically occur. It will also separate 3He from 4He down to
levels of ~1% and resolve Ne and Mg isotopes. In addition, the broad dynamic range
will provide measurements of trans-Fe elements with 30 < Z < 83, which are often
enriched by factor of ~100 in impulsive SEP events. LET accomplishes these
measurements using well-known ?E-E method of particle detection in six small stacks
of silicon detectors. Using the energy-loss patterns in successive detector layers, the
charge, mass, and kinetic energy of species from H to Ni can be uniquely identified onboard
at event rates up to ~10 kHz. The LET design has heritage from SOHO/ERNE,
which has more than 14 years of successful operation in space.
The prerequisites for particle identification are that the path-length variations of the ions
in the detectors and the electronic noise are sufficiently low. To fulfil these two
requirements, the angle of incidence of accepted particles is limited to ±20°, the first
two detectors are segmented, the uniformity of the solid state detectors is maintained
down to <0.5 %, and a high-performance front-end ASIC is utilized. In order to achieve
comprehensive angular coverage for resolving particle pitch-angle distributions, the
sensor unit has three view cones consisting of three telescopes with angular separations
of 60°. There are two identical sensor units to provide 3-D anisotropy information both
in and out of the ecliptic plane.
The basic structure of LET is shown in the Figure. Two circular front detectors, D1 and
D2, define the view cones of the three telescopes. The thickness of the D1 detector is 20
µm, while D2 is 80-µm thick. The thickness of D1 is the main factor determining the
lower limit of the operational energy range of LET and it also determines the proton
upper energy range due to the decreasing signal with increasing energy. For particle
identification, it is required that the particle penetrates the thin foil (e.g., 8 µm
polyimide with aluminium coating) above D1, the detector D1 itself, and gives a signal
(energy loss of ~150 keV) from D2. Below D2, the three silicon detectors, D3, D4, and
D5 have sufficient thickness to stop all particles with energies below the defined higher
limit of the operational energy range. At the bottom of each telescope, a detector (AC)
is used in anticoincidence with the other detectors to reject penetrating particles.
The front detectors D1 and D2 are both divided in three active areas, a centre area and a
surrounding annular area with two segments. This division gives angular resolution
within the 40° view cone of the three telescopes. More importantly, the field of view for
proton and helium measurements in each of the three directions can be dynamically
limited to a cone with a full opening angle of 14°, while simultaneously maintaining the
full opening angle for heavy ion measurements. This feature allows high-sensitivity
measurements of even small solar particle events, while ensuring unsaturated operation
during very high flux conditions of large SEP events. The total and centre area
geometric factors are 0.21 cm2sr and 0.0024 cm2sr, respectively, giving total geometry
factors of 1.26 and 0.014 cm2sr for six telescopes. The collecting power of the first
detector alone, determined by the active surface of D1 and the edge of the collimator
structure, is 1.68 cm2sr.
The analyzed data are transferred to the ICU once per second, which allows
implementation of a burst mode with 1 second cadence. However, the basic time
resolution of LET is 10 s. Proton, 3He, and 4He energy spectra and samples of PHA data
are accumulated by the ICU software with this cadence. A longer integration time of
100 s is used for the spectra of heavier ions.
