X - ray fluorescence is a phenomenon caused by changes in the atom. A stable atomic structure consists of the nucleus and the electrons outside the nucleus. The hewaidianzai run in their own energy in a fixed orbit respectively, the inner electrons (such as K) released from the atoms bound, the X ray energy sufficient under the electron ejected causes the electron shell phase should electron vacancy. At this time the electrons in the high energy electron shell (such as the L layer) will jump to the low energy electron shell to fill the desired electron vacancy. Due to the energy gap between different electron shells, the difference between these energies is released in the form of two X rays. The two X rays emitted by different elements have specific energy characteristics. This process is what we call X - ray fluorescence (XRF).