(4) When a K shell electron is ejected, the vacancy need not be filled from
the L shell. It can be filled from any shell with a higher potential energy level or even
from outside the atom. In any case, the emitted photon is always equal to the energy of
the difference of the transition. The combined energies of the photons from a collisional
interaction are equal to the binding energy of the shell from which the electron was
ejected because the atom initially absorbed that amount of energy. The maximum
energy of a photon cannot be greater than the binding energy of the K shell of the atom
(in the case of tungsten, 69.5 keV).
(5) The radiation produced in a collisional interaction is called characteristic
radiation because its energy is characteristic of the shell of the atom from which it came.
The binding energy of the K shell in copper is 9 keV; therefore, the maximum energy of
characteristic radiation that could be generated in copper would be 9 keV, an amount
that is not usable in radiology. Tungsten, however, can generate characteristic radiation
with a maximum energy level of 69.5 keV, some of which can be useful in radiology.
(6) In order for characteristic radiation to be generated at all, an electron
must be ejected from its shell. The energy required to remove an electron from its shell
is equal to or greater than the binding energy of the shell. Since the binding energy of
tungsten's K shell is 69.5, it would require an electron with at least 69.5 keV of energy to
eject the K electron and thus generate useful characteristic radiation from tungsten. It
should be noted at this time that 12 keV of electron energy can eject an L electron from
tungsten, thereby producing characteristic radiation. However, this radiation's maximum
energy would be 12 keV, which is not considered useful in radiology.
c. The Electromagnetic Spectrum. Gamma radiation and x-radiation are two
of several types of electromagnetic radiation that make up the electromagnetic
spectrum illustrated in figure 1-17. Notice that visible light waves, radio and television
waves, and infrared waves are also electromagnetic radiation. The main difference
between visible light and x- radiation is in their wavelength and, consequently, their
d. Wavelength, Energy, and Frequency. Electromagnetic waves, including
x-rays, all travel at the same speed in a vacuum--about 186,000 miles per second.
They have the same general characteristics, but differ in the length of the waves. This
is illustrated in figure 1-18, which shows two different wavelengths. The longer
the beam. X-rays have quite short wavelengths as compared to many other
electromagnetic waves, but they do vary somewhat. Aluminum filters are used in x-ray
machines to filter out the longer, less penetrating rays which otherwise would expose
the patient to radiation with an energy level too low to be useful in radiography.