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 Photo of Mars Pathfinder with APXS highlighted in red |
- Alpha particles (4He) are emitted from Curim source
- Using Rutherford’s laws for backscattering, the atomic mass of the nuclei can be determined
- Lighter elements absorb energy of alpha particle
- Heavier elements reflect energy of alpha particle of the same energy
- Most useful in determining lighter elemental composition of a sample
Proton*
- Alpha particles are absorbed by atomic nuclei producing protons of a defined energy
- Sodium, magnesium, silicon, aluminum, and sulfur detection
- Alpha particles ejected electrons from the inner electron shell (K or L shell)
- K series: ejection from K shell
- L series: ejection from L shell
- Outer shell electrons drop down to fill vacancies emitting energy.
- Only certain transitions are allowed.
- Energy emitted is characteristic of an element:
Wavelength =
K/E
where E is the energy emitted & K
is a constant = 1.9865 x 10-16 nm
J
- The number and energy of X ray emitted are measured
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Results
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 X-Ray
spectrum from
Barnacle Bill on Mars exhibiting a
diverse chemical composition (Reider 1997). |
References
R. Rieder, T. Economou, H. Wänke, A. Turkevich, J. Crisp, J. Brückner, G. Dreibus, H. Y. McSween Jr. "The Chemical Composition of Martian Soil and Rocks Returned by the Mobile Alpha Proton X-ray Spectrometer: Preliminary Results from the X-ray Mode." Science (1997). 278:1711-1774
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