Characteristic x-rays are produced due to

Characteristics X-ray are produced when an element is bombarded with high energy particles which can be photons, electrons or ions. When the incident particle strikes a bound electron in an atom, the bound electron is ejected from the inner shell of the atom. After the electron has been ejected, the atom is left with vacant energy level, also known as core hole. Outer shell electrons then fall into the inner shell, emitting quantized photons with an energy equivalent to the energy difference between the higher and lower states. Each element has a unique set of energy levels, and thus the transmission form the higher to lower energy level so produce X-rays with frequencies that are characteristics to each element.