The final way in which photons interact is another one of those Alice in Wonderland events that may seem impossible to believe. A photon with energy greater than 1 million electron volts (MeV) (in the presence of a nucleus to con­serve momentum) gives up all of its energy to create two new particles, a nega­tive electron and a positive electron (positron), according to Einstein’s equation, E = mc2 (Figure 7.3). It takes slightly over 1 MeV of energy to create the mass of two electrons. Any excess energy in the original photon is given up as kinetic energy of the electrons. The positive electron rapidly loses energy through col­lisions and finally engages in a dance of death with a regular negative electron; then they annihilate each other, creating two у photons, each with energy of 511 keV moving in opposite directions, thus converting mass back into pure energy. Energy becomes mass becomes energy! Ah, the strange things that happen in the atomic world!

The net effect of all of these types of electromagnetic interactions is that the X-rays and у rays are absorbed by matter in an exponential manner. That is, the number of X-rays or у rays and their dose decrease exponentially as they pass through matter (Figure 7.4). The more energetic they are, the farther they will travel, but even low energy X-rays used in radiology (around 70 to 100 keV) have plenty of energy to travel several inches through your body. The photons give up their energy by ionizing atoms and giving energy to electrons. It is the electrons that contribute most of the dose from energetic photons.