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. An x-ray fluorescence (XRF) spectrometer is used for routine, non-destructive, chemical analysis of rocks, mineral sediments, and fluids. It works on wavelength-dispersive spectroscopic principles that are similar to those of an electron microscope.