When the energy of the gamma photon (hv) is higher than the energy equivalent with the rest mass of two electrons (2m0c2), the gamma photon can transform into an electron and a positron when it passes the nuclear field. This process is called “pair formation,” the reverse process of annihilation (as discussed in Section 5.3.3). On the basis of the conservation of energy:

hv = m0c2 + Ee — + m0c2 + Ee+ (5.90)

where Ee — and Ee+ are the kinetic energy of the electron and the positron, respectively.

The cross section of the pair formation can be described as:

^ = KZ2f (Ey) (5.91)

where f(EY) is a factor depending on the energy of the gamma radiation, Z is the atomic number of the interacting substance (absorber), and K is constant. As seen, the cross section of the pair formation increases as the gamma energy and the atomic number increase.