(2) Compton effect. The Compton effect, also referred to as modified or
incoherent scattering, is the result of a partial transfer of energy from an x-ray photon to
an orbital electron, as seen in figure 4-5. In this case the photon strikes a glancing blow
to the electron and ejects it from orbit. Although considerably weakened in energy
(longer wavelength), the photon will continue on. While the now "soft" photon will
eventually disappear via a final photoelectric effect, the ejected electron can, as in the
previous case, continue on to cause another, or secondary, ionization of a nearby atom.
In contrast to the photoelectric effect, the Compton effect increases with increasingly
higher keV levels, becoming the dominant type at levels above 70 keV.
Figure 4-5. Comptom effect.
X-ray photons also undergo other interactions in matter. Two of them are
described briefly here, although they do not occur with photons in the diagnostic energy
a. Thomson Effect. The Thomson effect, also known as unmodified, classical,
and coherent scattering, is the result of a photon interacting with a whole atom as
shown in figure 4-6. This interaction results in the deflection or scattering of the photon
from its original direction without a loss of energy to the atom. The Thomson effect
occurs with photons of only a few kiloelectron volts of energy.
b. Pair Production. With gamma and x-radiation photons of energies greater
than 1.02 MeV, pair production occurs. At such energy levels, it is possible that all
three interactions can take place in a matter of microseconds. But pair production is
predominant among photons with energy in the 5- to 12-MeV range.