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 mass– Alpha particle – Protons– X-Ray

• Radioactive Currium is used as the alpha particle source

Photo of Mars Pathfinderwith APXS highlighted in red

Alpha Particles

• 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

Protons

• 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 allowed.

• Energy emitted.

Particle Induced X-Ray Emission

• Energy emitted is characteristic of an elementE = 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

measured

Results• 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 meteorites.

• Analysis indicates that sample rocks are andesitic (volcanic)

Advantages• Compact• Low power• Non destructive to sample• High sensitivity• High resolution (especially for heavier

elements)

Disadvantages• Slower than other available methods• Use of radioactive material