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GAMMA RAY SPECTROMETER

An introduction to the gamma ray spectrometer

GAMMA RAY SPECTROMETER

Gamma Ray Spectrometer
Images courtesy of Lockheed Martin and Los Alamos National Laboratory

The Gamma Ray Spectrometer (GRS) will map the abundances of ten elements on the Moon's surface: thorium, potassium, uranium, iron, oxygen, silicon, aluminum, calcium, magnesium, and titanium. It will be especially sensitive to the heavy, radioactive element thorium and the light element potassium. These are particularly plentiful in the Moon's youngest rocks, the last part of the crust to solidify. The data produced by the GRS will help scientists understand the origins of the Lunar landscape, and may also tell future explorers where to find useful metals like aluminum.

A gamma ray is a very energetic photon (a tiny parcel of light) -- more energetic than a visible light ray or an X-ray. The gamma rays that the spectrometer will detect come from two sources. "Natural" gamma rays are emitted spontaneously by radioactive elements like thorium and uranium. "Induced" gamma rays are emitted by elements like iron, silicon, and oxygen on the Moon's surface when they are bombarded by cosmic rays. The energy of a gamma ray serves as a distinctive signature of the atom that emits it.

When the gamma rays hit Lunar Prospector, they will pass through a crystal of bismuth germanate in the GRS. The bismuth atoms give off a flash of light when the radiation hits them; the more intense the gamma ray is, the brighter the flash. The energy of a gamma ray, in turn, tells researchers exactly which kind of atom emitted it.

The GRS will also contribute indirectly to the search for water on the Moon. The bismuth germanate crystal is surrounded by a shield of borated plastic that will detect epithermal neutrons.

The gamma ray energy spectrum has a lot of "noise" in it, due to gamma rays that collide with other atoms in the Moon's crust. Unlike the NS, the GRS will search for gamma rays that have escaped straight into space, because those are the ones that carry the telltale atomic signatures. Because of the uncertainty in these counts, one sweep over the Moon's surface would not give scientists enough information to determine the concentration of the radioactive elements. In addition, the stable elements do not emit gamma rays as readily as the naturally radioactive ones, so it will take Lunar Prospector up to a year or more to collect enough data to estimate their concentrations.

Lunar Prospector will pass over each "resolution element" -- that is, each 94-mile (150-kilometer) by 94-mile (150-kilometer) area on the Moon's surface -- about nine times per month (more frequently for polar regions). As they receive more data, scientists will be able to reduce the errors due to random noise or small numbers of gamma rays, using statistical analysis. In approximately three months, they should be able to generate useful estimates of the abundance of thorium and potassium. For aluminum, calcium, and magnesium, it may take up to the full 12 months.

* The Gamma Ray Spectrometer weighs 19 pounds (8.6 kilograms), uses 3 watts of power, and produces data at a rate of 688 bits per second. It will be deployed on one of Lunar Prospector's three booms.

 

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