The Synchronous Orbit Particle Analyzer (SOPA) consists
of three nearly identical silicon solid state detector telescopes, pointed
at 30, 90, and 120 degrees to the satellite's earth-centered spin axis.
The working end of the telescope consists of a very thin front silicon
detector, D1, followed by a thick back detector, D2. The D1 sensors are
mounted with the thin aluminum contact out. The thinner than usual Al
contact was chosen to minimize the entrance deadlayer to allow as low a
proton threshold as possible. Measurements show that the deadlayer is
approximately 30 micrograms/cm2. The detector stack is surrounded,
except for the aperture, by passive low-Z (aluminum) and high-A (copper)
shielding, which excludes side-penetrating protons up to about 65 MeV and
electrons up to 6 MeV. The front collimator baffle is designed to require
at least two-fold scattering of particles not in the acceptance angle of
the detector to encounter the D1 detector. This provides an extremely
sharp angular cutoff of incident particles. The full acceptance angle of
the telescopes is about 11 degrees. Each telescope has a geometrical
factor of 8.49 E-4 cm2 sr for ions, and 1.09 E-3 cm**2 sr for low-energy
electrons. A single rotation requires about 10 s, and in that time,
64 cuts of the unit sphere are taken by the three telescopes in a certain
pattern. Passive cooling keeps the telescope temperatures within the range
-15 to +5 C. In this range, essentially all of the noise associated with
leakage current in the surface barrier detectors is eliminated. Even so,
the high capacitance of the D1 sensors (~500 pF) requires setting the
first energy threshold relatively high. The numerous thresholds and logic
channels (including anti-coincidence) result in identification of
differential fluxes of protons from 50 KeV to 50 MeV. Differential fluxes
of electrons are determined from 50 KeV to 1.5 MeV, with integral flux
above 1.5 MeV. The differential flux range for alpha particles is
0.5--1.3 MeV; for CNO, 1.5--3.55 MeV; for carbon, 5.0--13 MeV; for
nitrogen, 6.0--14 MeV; and for oxygen, 7.2--15 MeV. Integral ion fluxes
are determined for sulphur above 8 MeV, and for bromine above 15 MeV.
A sun sensor with three collimators that overlap the fields-of-view of
the three telescopes provides input to tag the pulse pairs that are
potentially contaminated by sun-generated pulses from D1. For more
details of the instrument, see the paper by R. D. Belian et al.,
J. Geophys. Res., 97, A11, pp. 16897-16906, 1992.