T.O. 33B-1-16-86.2.2.3 ImpingementofElectronsonaTarget.6.2.2.3.1 ContinuousX-RaySpectrum.Merely generating electrons in a vacuum and setting them in motion is not sufficient to create X-rays. It is necessaryalso that the electrons strike some target substance. In an X-ray tube the target is the anode. When the electronsbombard the target, they are brought to an abrupt halt. Unfortunately most of the electrons’ kinetic energy is convertedinto heat which must be dissipated by the target material. Only a small percentage of the energy available in theelectron beam is converted into X-ray photons which can have energies ranging from zero to a maximum which isdetermined by 1) the original kinetic energy of the electrons and 2) by how rapidly the electrons are decelerated. Thisprocess produces the continuous portion of the X-ray spectrum and is known either by the German termBremsstrahlung, meaning braking radiation, or by the term white radiation (see paragraph 6.5.3). X-rays are producedregardless of the material bombarded, whether it is a solid, liquid or gas. In the X-ray tube a solid material is used forthe target. The higher the atomic number of the target material the higher the efficiency of X-ray production.6.2.2.3.2 CharacteristicX-RaySpectrum.In addition to the white radiation, there are several characteristic peaks in a typical X-ray spectrum. These intensityspikes are caused by interaction between the impinging stream of high-speed electrons and the electrons that are boundtightly to the atomic nuclei of the target material. If the atom is considered as a planetary system with the nucleus ofprotons and neutrons at the center of the system and the electrons moving in orbits around the nucleus, modern physicspredicts that the orbital electrons near the nucleus will have very well-defined energies, with electrons in differentorbits having different energy levels. If an electron from an external beam collides with one of these orbital electronswith sufficient energy to knock it out of its orbit, an electron from a higher energy level would, after a time, drop downto fill the void and restore atomic stability. When that electron drops to the lower energy level, it gives off a photonwith energy equal to the difference in energy levels. Since these energy levels depend strictly upon the particular atom,the radiation emitted is called characteristic radiation. The characteristic radiation emitted by the target material issuperimposed upon the continuous spectrum. A typical X-ray spectrum of radiation generated by an X-ray tube wouldappear as shown in Figure 6-6. The K and L series of characteristic radiation designates the radiation emitted fromdifferent electron orbits around the nucleus of the atom. As energy levels increase, electrons are dislodged from thevarious orbits with the K series being the closest to the nucleus.Figure 6-6. Typical X-ray Spectrum.
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