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The Charged Particle Analyzer (CPA) experiment consists of four
detectors including the LoE, HiE, LoP and HiP. Each will be
described in some detail. The reader is referred to Higbie, et
al., 1978, or Baker, et al., 1985, for further instrument
information.
CPA experiments were flown on a series of LANL satellites
beginning in 1976. In all, 8 satellites were flown, the last was
launched in 1987. After 1979 a constellation of at least three
satellites with CPA detectors were continuously maintained and
the data from some subset were processed up until
1995. Currently two satellites remain operational, however, the
CPA data from these is not processed due to processor
limitations and their function has been replaced with other LANL
satellites with SOPA detectors.
The LoE (low energy electron) subsystem is a set of five,
similar, solid-state sensors (700 microns thick) in a fan
arrangement at angles of +/- 60, +/- 45 and 0 degrees to the
normal to the satellite spin axis (directed toward the
Earth). Each telescope has a collimating aperture with a half
angle of ~2.6 degrees, which provides a geometrical factor of
3.096E-03 cm-sq sr. As the satellite spins, (period of ~10
seconds), the telescopes sweep out parallel bands. During the
course of a single rotation, the five LoE detectors record 200
samples of the unit sphere at each energy. There are six nested
energy channels with approximate lower thresholds of 30, 45,
65, 95, 140, and 200 keV. Each channel has an upper threshold
of 300 keV. Each of the five telescopes is fronted by an
aluminized Mylar sunscreen. The LoE telescopes do not
discriminate between particle species, but rely instead on the
normal preponderance of electrons at GEO. Therefore, there is
always a small contaminatio n of each electron channel by
protons and other ions (usually less than 1%).
The HiE (high energy electron) subsystem is a single element,
collimated, solid-state sensor (3000 microns thick) mounted
perpendicular to the spacecraft spin axis with a field-of-view
half angle of 3.7 degrees and a geometrical factor of 1.407 E-02
cm-sq sr. It is designed to monitor high-energy electrons. There
are six nested energy channels with approximate lower thresholds
of 200, 290, 430, 630, 930, and 1350 keV. Each channel has an
upper threshold of 2000 keV. The HiE detector records 40 samples
of the particle distribution each rotation. It is fronted by an
aluminized Mylar sunscreen. As with the LoE, the HiE depends on
the preponderance of electrons at GEO for species
discrimination.
The LoP (low energy proton) subsystem is a single element,
collimated, solid-state sensor (85 microns thick) mounted
perpendicular to the spacecraft spin axis with a field-of-view
half angle of 2.8 degrees and a geometrical factor of 2.948E-03
cm-sq sr. It is designed to monitor protons, and has 10 nested
energy channels with lower thresholds that vary from satellite
to satellite, but which are about 80, 90, 110, 135, 175, 200,
240, 300, 365, 455 keV and an upper threshold of ~580 keV. The
LoP detector records 40 samples of the particle distribution
each rotation. Electron contamination to the proton channels is
minimized by the action of a sweeping magnet. An
anti-coincidence scintillation discriminator vetoes particles
penetrating the sensor. The LoP is fronted by a 90 micro-inch,
nickel sunscreen.
The HiP (high energy proton) subsystem is a single, tri-element,
collimated, solid-state telescope (45 microns, 3300 microns,
and 500 microns thick) mounted perpendicular to the spacecraft
spin axis. Except for the collimated aperture, the stack of
three solid-state sensors is completely surrounded by a plastic
anti-coincidence shield. The HiP detector has two slightly
different fields-of-view and geometrical factors, depending on
which sensor element the particle is analyzed by field-of-view
half angles of 6.5 and 6.7 degrees and geometrical factors of
4.685 E-02 and 4.813 E-02 cm-sq sr. The HiP is designed to
monitor high-energy protons with 16 differential energy
channels. The channels differ slightly from satellite to
satellite due to slight variations in sensor dead-layers and in
the thickness of the front elements. However, the nominal
energy thresholds for the 16 channels are 0.4 MeV, 0.5, 0.6,
0.8, 1.0, 1.3, 1.7, 2.8, 4.3, 8.0 14, 23, 33, 48, 71, and 100,
with an uppe r threshold of ~160 MeV. Like the LoP, the HiP
records 40 samples of the particle distribution each
rotation. Again, electron contamination to the proton channels
is minimized by the action of a one kilogauss sweeping
magnet. The HiP collimator is fronted by an aluminized Mylar
sunscreen.
[Summary provided by Leadbelly.]
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