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Alpha Particle Spectrometer
Image courtesy of
Los Alamos National Laboratory

Prospector's Alpha Particle Spectrometer (APS) will detect alpha particles emitted by radioactive gases, such as radon and polonium, leaking out of the lunar interior. While the Moon currently lacks volcanoes, it does appear to vent gases such as radon , nitrogen, and carbon dioxide. The APS will search for these outgassing events by detecting alpha particles. An alpha particle is essentially the nucleus of a helium atom: two protons and two neutrons bound together. Like gamma rays, alpha particles escape from radioactive elements as part of their natural decay process. The alpha particles are emitted with a precise energy that serves as a fingerprint for the atom from which they came. Inside the APS are ten separate wafers of silicon. Silicon, a semiconductive material, conducts electrical charge only minimally. For that reason, however, it produces a high-resolution signal, since it effectively blocks out most extraneous, background charge. When an alpha particle hits a silicon wafer, it creates a small track of charge. When a high voltage is applied to the silicon wafer, the alpha particle's charge is funneled into an amplifier (an aluminum disk atop the silicon), where it is collected. Since that pulse of charge is directly proportional to the signature energy of the alpha particle, scientists can infer the identity of the element which emitted the alpha particle.

The APS instrument contains ten such silicon detectors, each sandwiched between gold and aluminum disks, and arranged on five out of six sides of a cube, enabling nearly a complete field of detection.

The detection of gases will depend very much on whether any outgassing events occur while Lunar Prospector is in orbit, and how many there are. Of course, the longer the mission lasts, the more events scientists are likely to see. Also, because the outgassing events are localized, the precision of the data will improve considerably if, as currently planned, Lunar Prospector has enough fuel after the first year to drop to a lower orbit 6 miles above the surface. (For a rough terrestrial analogy, from the higher orbit Lunar Prospector would be able to say that a volcano has erupted in the state of Washington. >From the lower orbit, it would be able to say which volcano.)

* The Alpha Particle Spectrometer weighs 9 pounds (4 kilograms), consumes 7 watts of power and produces data at a rate of 181 bits per second.

Go to Alpha Particle Spectrometer Results | Go to Instrument Introduction