NEAR's X-Ray/Gamma-Ray Spectrometer

X-Ray/Gamma-Ray Spectrometer

The X-ray/Gamma-ray Spectrometer (XGRS) on the spacecraft of NASA'a NEAR mission will measure and map the abundance of various elements on the surface of Eros. High-energy X-rays from the sun strike the sunlit part of Eros's surface, and cause different elements to give off characteristic spectra of lower-energy X-rays. These characteristic spectra serve as "fingerprints" of the elements. At visible wavelengths, this would be the equivalent of distinctive colors. Given Eros's expected rock-metal composition, the major elements to be detected will be magnesium, aluminum, silicon, calcium, iron, and possibly sulfur and titanium depending on Eros's exact makeup. Intensities of the different elemental spectra serve as indicators of the elements' relative abundances.

Gamma-rays, which characteristically have shorter wavelengths, will be used to measure elements approximately 10 to 20 cm below Eros' surface. These gamma rays arise partly from natural radioactivity, and partly from stimulation of specific elements by high-energy cosmic rays. The naturally radioactive elements include potassium, thorium, and uranium; other elements measured include hydrogen, oxygen, silicon, and iron.

The assembled instrument on the spacecraft has many components, some of which are shown below.

[Flight X-Ray Instrument] [X-Ray Proportional Counter]

[Flight Gamma-Ray Sensor Head] [X-Ray Solar Monitor]

Lesson Plans on the Electromagnetic Spectrum

XRGS Overview on NEAR Web Server

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