Figure 4-3. Ionization of an atom.
c. The two types of ionization that generally occur in the diagnostic radiation
energy range are the photoelectric effect and the Compton effect.
(1) Photoelectric effect. The photoelectric effect, illustrated in figure 4-4, is
an all-or-none energy exchange in that the photon strikes an electron, imparts all of its
energy to it, and simply vanishes. The electron ejected in this way is called a
photoelectron. It departs with all the inherent energy of the photon and can cause
secondary ionization due to its increased kinetic energy. In the meantime, as the
excited atom returns to the normal state, it quickly attracts another electron to fill the
vacant "hole," and radiation is emitted. The energy of the radiation (and the sequence
of events that causes the radiation) is much the same as the replenishment of an
electron shell vacancy created by the ejection of an electron by electron collision as
explained earlier in this subcourse. The photoelectric effect normally occurs with
photon energies up to 100 keV.
Figure 4-4. Photoelectric effect.