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Synchronous Orbit Particle Analyzer (SOPA)

NSSDCA ID: 1991-080B-02

Mission Name: USA 75
Principal Investigator:Mr. Richard D. Belian


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. Similar instruments are on board all three of the constellation of geosynchronous spacecraft 89-046A, 90-095A, and 91-080B. 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/cm**2. 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 cm**2 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, from which this information was obtained.

Alternate Names

  • SOPA
  • USA75/SOPA

Funding Agency

  • Department of Energy (United States)


  • Space Physics: Magnetospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Dr. H. Kent Hills



NameRoleOriginal AffiliationE-mail
Mr. Richard D. BelianPrincipal InvestigatorLos Alamos National

Selected References

  • Belian, R. D., et al., High-Z energetic particles at geosynchronous orbit during the great solar proton event series of October 1989, J. Geophys. Res., 97, No. A11, 16897-16906, doi:10.1029/92JA01139, Nov. 1992.
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