At higher energy levels, pair production is predominate. When a high energy gamma passes
close enough to a heavy nucleus, the gamma disappears, and its energy reappears in the form of
an electron and a positron (same mass as an electron, but has a positive charge), as shown in
Figure 4. This transformation of energy into mass must take place near a particle, such as a
nucleus, to conserve momentum. The kinetic energy of the recoiling nucleus is very small;
therefore, all of the photons energy that is in excess of that needed to supply the mass of the
pair appears as kinetic energy of the pair. For this reaction to take place, the original gamma
must have at least 1.02 MeV energy.
Figure 4 Pair Production
The electron loses energy by ionization. The positron interacts with other electrons and loses
energy by ionizing them. If the energy of the positron is low enough, it will combine with an
electron (mutual annihilation occurs), and the energy is released as a gamma. The probability
of pair production increases significantly for higher energy gammas.
Gamma radiation has a very high penetrating power. A small fraction of the original stream will
pass through several feet of concrete or several meters of air. The specific ionization of a gamma
is low compared to that of an alpha particle, but is higher than that of a beta particle.