Characteristic x-ray

A high energy electron interacts with a bound electron in an atom and ejects it. The incident electron is scattered and the target electron gets displaced from its shell. The incident electron energy must exceed the binding energy of the electron to eject it. After the electron has been ejected the atom is left with a vacant energy level.

This vacant energy level if it occurs in the inner electron levels is called a core hole. This vacancy is subsequently filled by an electron from a higher energy level with the emission of a characteristic x-ray photon. The characteristic x-ray photon has an energy that corresponds exactly to the difference in energy between the energy level that is vacant and the energy level from which an electron falls.

The x-ray energy is characteristic of the atom that had the core hole and can be used to identify the atom. This is used in various techniques, including X-ray fluorescence spectroscopy, Energy dispersive X-ray spectroscopy and Wavelength dispersive X-ray spectroscopy. These are used in mineral analysis and elsewhere.

The characteristic-x-ray can be immediately reabsorbed by an electron in the same atom and instead the energy of the characteristic x-ray is given entirely to this electron which is called an Auger electron. This is like an internal photo-electric effect that occurs after the initial photo-electric effect.

References

• Khan F The physics of radiation therapy