2.5.3.1. The Origin of Characteristic X-rays

When a sample is bombarded by an electron beam, some electrons are knocked out of their shells in a process called inner-shell ionization. About 0.1% of the electrons produce K-shell vacancies; most produce heat. Outer-shell electrons fall in to fill a vacancy in a process of self-neutralization (Figure 2.5.3.1). The energy required to produce inner-shell ionization is termed the excitation potential or critical ionization potential (Ec).

C.G. Barkla (1877-1944)

The production of "characteristic" X-rays by electron bombardment of pure elements was first observed in 1909 by Charles G. Barkla and C.A. Sadler. However, the physical origin of X-rays was not clear. Barkla received the Nobel Prize in 1917.

 

Models showing production of bremsstrahlung, characteristic X-rays, and Auger electrons

Figure 2.5.3.1. Classical models showing the production of bremsstrahlung, characteristic X-rays, and Auger electrons. (left) Electrons are scattered elastically and inelastically by the positively charged nucleus. The inelastically scattered electron loses energy, which appears as bremsstrahlung. Elastically scattered electrons (which include backscattered electrons) are generally scattered through larger angles than are inelastically scattered electrons. (right) An incident electron ionizes the sample atom by ejecting an electron from an inner-shell (the K shell, in this case). De-excitation, in turn, produces characteristic X-radiation (above) or an Auger electron (below). Secondary electrons are ejected with low energy from outer loosely bound electron shells, a process not shown.


When outer-shell electrons drop into inner shells, they emit a quantized photon "characteristic" of the element. The energies of the characteristic X-rays produced are only very weakly dependent on the chemical structure in which the atom is bound, indicating that the non-bonding shells of atoms are the X-ray source. The resulting characteristic spectrum is superimposed on the continuum. An atom remains ionized for a very short time (about 10-14 second) and thus the incident electrons that arrive about every 10-12 second can repeatedly ionize an atom. However, not all outer-shell electrons can fall in to produce X-rays.

 

To more fully understand the production of characteristic X-rays, we must first consider the electronic structure of atoms.

 


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Last update: 01/18/2006 01:47 PM.