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Non Destructive Testing Methods Part 3 – Positive Material Identification

April 17, 2012

Positive Material Identification (PMI) is a nondestructive means of determining the chemical composition of metals. In a global sourcing environment, material can inadvertently be mixed up and cause serious issues while in service.

X-ray fluorescence (XRF) is the emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science and archaeology.

Underlying physics

When materials are exposed to short-wavelength X-rays or to gamma rays, ionisation of their component atoms may take place. Ionisation consists of the ejection of one or more electrons from the atom, and may take place if the atom is exposed to radiation with an energy greater than its ionisation potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way renders the electronic structure of the atom unstable, and electrons in higher orbitals “fall” into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The termfluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy (generally lower).

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