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Lunar Prospector Alpha Particle Spectrometer (APS)

NSSDCA ID: 1998-001A-03

Mission Name: Lunar Prospector
Principal Investigator:Dr. Alan B. Binder

Description

The Alpha Particle Spectrometer (APS) is designed to detect radon outgassing events on the surface of the Moon. The APS will record alpha particle signatures of radioactive decay of radon gas and its daughter product, polonium. These putative outgassing events, in which radon, nitrogen, and carbon dioxide are vented, are hypothesized to be the source of the tenuous lunar atmosphere, and may be the result of low-level volcanic/tectonic activity on the Moon. Information on the existence, timing, and sources of these events will help in a determination of the style and rate of lunar tectonics.

The APS is a cube approximately 18 cm on a side colocated with the neutron spectrometer on the end of one of the three radial 2.5 m Lunar Prospector science booms. It contains ten 3 x 3 cm square ion-implant silicon detectors covered by thin, aluminized (2000 angstroms) polypropylene foils arranged on five of six sides of the cube. The sensors were fully depleted to a depth of 55 mu to reduce the proton background in the 4.1 to 6.6 MeV range of radon decay alpha particle lines. The detectors are collimated ot a 90 degree field of view, full width at half maximum, have a total coverage of 3.5 pi sr, and a spectral energy resolution of about 100 keV at 5.5 MeV. Alpha particles, produced by the decay of radon and polonium, leave tracks of charge on the silicon wafers when they impact the silicon. A high voltage is applied to the silicon, and the current is amplified by being funnelled along the tracks and is recorded for identification by a charge sensitive preamplifier (one of reach of the ten detectors). The preamplifier signals are sent to the APS front-end electronics board and the analog signals from all sensors are summed into a single amplifier. The APS will make a global examination of gas release events and polonium distribution with a surface resolution of 150 km and a precision of 10%.

The Alpha Particle Spectrometer takes advantage of the uranium-238 decay series. Radon-222 is a gaseous daughter product of the decay of uranium-238 with a decay alpha particle energy of 5.48 MeV and a half-life of 3.8 days. It decays into Polonium-210, a solid with a half-life of 3 minutes to decay to lead-214 and an energy of 6.0 MeV. The lead decays rapidly through a few more steps until it becomes lead-210, which has a half-life of 22 years. The lead-210 decays to bismuth-210, which, with a half-life of 5 days, decays to polonium-210. Polonium-210 has a half-life of 138 days and a decay alpha particle energy of 5.3 MeV. Radon-222 has time to spread out several hundred km over its 3.8 day half-life. Detection of radon-222 and polonium-210 alpha particles indicates recent outgassing or exposure from a uranium source within a few hundred km, while polonium-218 is indicative of outgassing which may have occurred in the past.

After launch, during separation of the Lunar Prospector spacecraft and the second stage rocket, a light leak developed in the APS which was handled by disconnecting the affected detector by ground command. A large amont of solar charged particle activity increased the background noise which complicated the analysis of the data.

Alternate Names

  • APS
  • LunarProspector/APS
  • urn:nasa:pds:context:instrument:lp.aps

Facts in Brief

Mass: 4 kg
Power (avg): 7 W
Bit rate (avg): 0.181 kbps

Funding Agency

  • National Aeronautics and Space Administration (United States)

Discipline

  • Planetary Science: Geology and Geophysics

Additional Information

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

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Alan B. BinderPrincipal InvestigatorLockheedabinder@mail.arc.nasa.gov

Selected References

  • Feldman, W. C., et al., Gamma-Ray, Neutron, and Alpha-Particle Spectrometers for the Lunar Prospector mission, J. Geophys. Res., 109, E07S06, doi:10.1029/2003JE002207, 2004.
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