Alpha Proton X-ray Spectrometry (APXS) and the Mars Pathfinder

Particle Induced X Ray Emission
and Mars Exploration Rovers
Alpha Particle X-ray Spectrometry
• APXS uses a variety of
radiation techniques to
determine atomic nuclei
– Alpha particle
– Protons
– X-Ray
• Radioactive Currium is
used as the alpha particle
Photo of Mars Pathfinder
with APXS highlighted in red
Alpha Particles
• Alpha particles (4He) are emitted from Curim
• 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
• Alpha particles are absorbed by atomic nuclei
producing protons of a defined energy
• Sodium, magnesium, silicon, aluminum, and
sulfur detection
• Used only by Mars Pathfinder, subsequent
Mar Exploration Rovers did not use
Particle Induced X-Ray Emission
• 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, but only certain transitions are
• Energy emitted.
Particle Induced X-Ray Emission
• Energy emitted is characteristic of an element
E = Energy difference between shells
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
• Rock sample from
Barnacle Bill in Ares
Vallis on Mars
• Unexpectedly high in
silica and potassium;
low in magnesium
compared to martian
soils and martian
• Analysis indicates
that sample rocks are
andesitic (volcanic)
Low power
Non destructive to sample
High sensitivity
High resolution (especially for heavier
• Slower than other available methods
• Use of radioactive material

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