Source
Several forms of radiation are used in APXS. They include alpha particles, protons, and X-rays. Alpha particles, protons, and X-rays are emitted during the radioactive decay of unstable atoms. A common source of alpha particles is curium 244. It emits particles with an energy of 5.8 MeV. X-rays of 14 and 18 keV are emitted in the decay of plutonium 240. The Mars Exploration Rovers' Athena payload uses curium 244 with a source strength of approximately 30 millicuries (1.1 GBq).
Read more about this topic: Alpha Particle X-ray Spectrometer
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