Characteristics Radiation

When a fast moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. This hole is filled by an outer shell electron (from the L-shell, M-shell etc) with an emission of a single X-ray photon, called characteristic radiation, with an energy level equivalent to the energy level difference between the outer and inner shell electron involved in the transition.

As opposed to the continuous spectrum of bremsstrahlung radiation, characteristic radiation is represented by a line spectrum. As each element has a specific arrangement of electrons at discrete energy levels then it can be appreciated that the radiation produced from such interactions is 'characteristic' of the element involved.

For example, in a tungsten target electron transitions from the L-shell to the K-shell produce X-rays photons of 57.98 and 59.32 keV. The two energy levels are as a result of the Pauli exclusion principle which states that no two particles of half-integer spin (such as electrons) in an atom can occupy exactly the same energy state at the same time, therefore the K-shell represents two different energy states, the L-shell eight states and so on.

 Actually, a given anode material gives rise to several characteristic x-ray energies. This is because electrons at different energy levels (K, L, etc.) can be dislodged by the bombarding electrons, and the vacancies can be filled from different energy levels.Although filling L-shell vacancies generates photons, their energies are too low for use in diagnostic imaging. Each characteristic energy is given a designation, which indicates the shell in which the vacancy occurred, with a subscript, which shows the origin of the filling electron. A subscript alpha (a) denotes filling with an L-shell electron, and beta ((3) indicates filling from either the M or N shell.

When an electron falls (cascades) from the L-shell to the K-shell, the X-ray emitted is called a K-alpha X-ray. Similarly, when an electron falls from the M-shell to the K-shell, the X-ray emitted is called a K-beta X-ray. However, it is possible to have M-L transitions and so on but their likelihood is so low they can be safely ignored.

Each element differs in nuclear binding energies and characteristic radiation depends on the binding energy of particular element.

In mammography X-ray tubes which typically use a molybdenum target, more than 80% of radiation is characteristic radiation. However, characteristic radiation never exists in isolation and the line spectra is usually superimposed on the continuous spectra of bremsstrahlung radiation.