FIGURE 1-2 Mean free path of photons and neutrons in water and range of electrons and protons. SOURCE: Data from ICRU (1970).

The positron ultimately interacts with another electron, and this results in an “annihilation” event in which the mass is extinguished and two 0.51 MeV photons are emitted in opposite directions. The annihilation photons can themselves produce further ionizations.

Figure 1-2 shows the mean free path for monoenergetic photons (i.e., the average distance in water until the photon undergoes an interaction). To compare the penetration depth of photon radiation with that of electron radiation, the mean range of electrons of specified energy is given in the same diagram. It is seen that the electrons released by photons are always considerably less penetrating than the photons themselves.

Figure 1-3 compares in terms of the distributions of photon energy fluence the γ-rays from the A-bomb explosions with the distributions of photon energy for orthovoltage X-rays and low-energy mammography X-rays. These different electromagnetic radiations are all classified as low-LET (i.e., sparsely ionizing) radiation. There are, nevertheless, differences in effectiveness and possibly also differences in the risk for late effects due to these radiations.

The passage of fast electrons through tissue creates a track of excited and ionized molecules that are relatively far apart. X- and γ-rays produce electrons with relatively low linear energy transfer, (i.e., energy loss per unit track length) and are considered low-LET radiation. For example, the track average of unrestricted LET of the electrons liberated by cobalt-60 (⁶⁰Co) gamma rays is about 0.25 keV/μm, which can be contrasted with an average LET of about 180 keV/μm for a 2 MeV α-particle, a high-LET radiation. LET is an important measure in the evaluation of relative biological effectiveness (ICRU 1970; Engels and Wambersie 1998) of a given kind of radiation.