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Lunar Particle Shadows and Boundary Layer

NSSDCA ID: 1971-063D-01

Mission Name: Apollo 15 Subsatellite
Principal Investigator:Prof. Kinsey A. Anderson

Description

This experiment was designed to study the plasma regimes through which the Moon moves, the interaction of the Moon and plasmas, and some features of the structure and dynamics of the Earth's magnetosphere. Specifically the experiment measured intensities and energy spectra of electrons and protons from lunar orbit. Two 2-element solid-state particle telescopes and four electrostatic analyzers were used. A virtually identical instrument was also flown on the Apollo 16 subsatellite.

The two solid state particle telescopes (A and B) were aligned along the spacecraft spin axis and measured six levels of electron and proton energies. Each telescope was 4.5 cm long and had a 4 cm base with a set of collimators and two detectors. The collimator was passive and consisted of 11 aluminum rings with knife edges mounted in a cylinder that defined a 15 degree half-angle entrance cone. (Some particles entering at up to 20 degrees could be counted due to the finite area of the detector.) All internal surfaces of the collimator were anodized to a flat black to protect the detectors from solar illumination. Below the collimator centered on the same axis was a disk-shaped 25 square millimeter Si(Li) fully depleted surface barrier detector. The detector was mounted with the active surface-barrier side away from the collimator making the positively biased aluminum-coated side the particle entrance surface. The opposite side was covered by 40 micrograms per square cm of evaporated gold. The detector had a thickness to stop electrons with less than ~300 keV and protons with less than 6000 keV. Behind this detector was a second detector with an area of 50 square millimeters placed in anti-coincidence with the front detector. The active surface barrier side faced the front detector. The rear detector required an energy loss of at least 20 keV. The arrangement will reject particles that enter at the acceptance angle of the telescope and emerge from the front detector and those that enter from other directions.

The two telescopes differed in that one (the B telescope) had a 375 microgram per square cm layer of organic (Parylene N) foil in front of the first detector in which incident electrons lost relatively little energy (~5 keV) and protons lost relatively much energy (~310 keV). Each telescope had its own analog signal processor. When a particle entered through the collimators and was stopped in the front detector the analog output passed to a stacked-discriminator pulse-height analyzer which was switched from one telescope to another. The electron thresholds were switched between telescopes to compensate for the loss in the foil. Each telescope was operated at six discrimination levels, which corresponded on both telescopes to electron threshold energies of 18.4 to 331.5, 40.2 to 83.3, 83.3 to 153.4, 153.4 to 331.5, 331.5 to 528, and 528 to 2000 keV. On high setting the first range changed to 21.4 to 331.5. The unshielded A telescope was sensitive to protons of energies 32 to 343 (36 to 343 on high), 56 to 99, 99 to 167, 167 to 343, 343 to 537, and 537 to 2000 keV. The shielded B telescope measured protons at 342 to 538 (345 to 538 high), 356 to 377, 377 to 416, 416 to 538, 538 to 696, and 696 to 2000. There were also two channels for calibration. Species resolution was determined from the relative responses of the two telescopes. The low-energy thresholds of the telescopes could be altered between low and high by ground command to account for thermal noise. A weak radioactive source (plutonium 239) was placed near the front detector in each telescope to test the instrument stability.

The four electrostatic analyzers were oriented perpendicular to the spacecraft spin axis and measured both large fluxes of electrons in the energy windows 0.53 to 0.68, 1.75 to 2.25, and 5.8 to 6.5 keV, and small fluxes of electrons in the windows 5.5 to 6.5 and 13.5 to 15.0 keV. These analyzers did not count protons. Spin-integrated counts were obtained for all energy windows except the 13.5 to 15.0 keV window in which four-sectored data were obtained.

Each electrostatic analyzer consisted of two nested hemispherical copper plates with a small space between them. The outer plate was grounded and the inner plate was raised to a positive potential. The entrance aperture between the paired plates is open on one end and exposed to space. An electron entering at a small enough angle with an energy within an interval determined by the plate radii and the bias voltage on the inner plate will travel the full 180 degrees within the plates to an electron detector at the exit aperture. The first analyzer, referred to as A1, used a Spiraltron one channel multiplier (C1) to detect intense fluxes of low-energy electrons (.53 to .68 keV). The second analyzer, A2, is geometrically identical to A1 but uses a different plate voltage to detect electrons of different energies (1.75 to 2.25 keV) with detector, C2, identical to C1. The third analyzer, A3, had two detectors, C3 and C4. C4 consisted of two 1 cm diameter funnel-mouth Spiraltron multipliers in parallel and measured electrons of energy 5.5 to 6.5 keV. The funnel mouth multipliers are surrounded by a plastic scintillator viewed by a photomultiplier, connected in anti-coincidence to eliminate cosmic-ray induced counts. C3 is one small aperture Spiraltron multiplier measuring electrons in the energy range 5.8 to 6.5 keV. The fourth and largest analyzer, A4 (detector C5) used 5 funnel-mouth Spiraltron multipliers in parallel surrounded by a plastic anti-coincidence shield to measure electrons from 13.5 to 15.0 keV. It was time-division multiplexed so that four sectors could be examined separately, these were defined as sector I (-45 to +45 degrees of the magnetic field vector), sector II (+45 to +90 and +270 to +315), sector III (+90 to +135 and +225 to +270) and sector IV (+135 to +225). Gold backing and serration of the inner surface of the outer plate prevents ultraviolet light from reaching the detectors.

The Apollo 15 subsatellite was deployed on 4 August 1971 at 20:30 UT and the analyzers were turned on on 5 August at 20:00 UT. On 19 August at 20:50 UT the pulse-height analyzer threshold was raised from low to high due to unexpectedly high temperatures and attendant thermal noise. After 5 October 1971 the A particle telescope malfunctioned intermittently due to an apparent temperature-dependent gain shift. Count rates in channels 1 and 4 are high and unreliable. The telescope appeared to recover by December 1971. Short periods of high noise, presumably heat induced, were occasionally seen on the B telescope. There were also two telemetry failures, on 4 and 22 February 1972. Over the lifetime of the experiment it was found that the B (shielded) telescope was often counting at rates in excess of the A (open) telescope, this was concluded to be due to high thermal noise causing a breakdown in the A anticoincidence detector, or less likely an upward gain shift in B or a downward gain shift in A. Ground support for the experiment was terminated on 22 January 1973.

Alternate Names

  • Apollo15Subsatellite/LunarParticleShadowsandBoundaryLayer
  • S173

Disciplines

  • Space Physics: Magnetospheric Studies
  • Planetary Science: Fields and Particles
  • Space Physics: Heliospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Mr. Patrick E. LaffertyGeneral ContactNASA Johnson Space Center
Dr. James E. McCoyOther InvestigatorNASA Johnson Space Centerjmccoy@ems.jsc.nasa.gov
Mr. L. M. ChaseOther InvestigatorUniversity of California, Berkeley
Dr. Gerald SchubertOther InvestigatorUniversity of California, Los Angelesgschubert@astemis.ucla.edu, gschubert@geovax.ess.ucla.edu
Prof. Robert P. LinOther InvestigatorUniversity of California, Berkeley
Prof. Kinsey A. AndersonPrincipal InvestigatorUniversity of California, Berkeleyanderson@ssl.berkeley.edu

Selected References

  • Anderson, K. A., et al., Subsatellite measurements of plasmas and solar particles, in Apollo 15 Prelim. Sci. Rept., NASA SP-289, Wash, DC, 1972.
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