Bioenvironmental Engineering Apprentice (BEA) Block 7 – Ionizing Radiation Practice Test

When a photon is energetic enough and passes near a nucleus, it can convert into matter because the energy has to go somewhere. In this process, the photon creates an electron and a positron, with the nucleus taking a small recoil to conserve momentum. The energy balance is that the photon’s energy becomes the rest-mass energy of the electron-positron pair plus their kinetic energy (and a tiny portion goes into the nucleus’s recoil). Since two electron masses are required, the photon must have at least 1.022 MeV of energy; any extra energy shows up as kinetic energy of the new particles (and the recoiling nucleus).

This is why the best description is that energy converts into the rest mass of two electrons and the remaining energy appears as kinetic energy of the electron and positron (and small recoil of the nucleus). Other scenarios—such as the photon being absorbed by the nucleus without creating particles, capturing a neutron and releasing gamma, or scattering elastically without producing new particles—don’t describe pair production.