We have discussed the interaction of the three primary types of ionizing radiation (alpha, beta, and gamma) and neutrons with water and with living tissue, which contains a great deal of water. Alpha radiation provides short, dense ionization tracks in water (or living tissue); beta radiation produces sparse tracks that are longer; and the highly penetrating gamma radiation leaves scattered local regions of ionization where the energetic photons have knocked electrons free from their atoms. These local regions have the same type of ion density as the tracks from beta particles. Finally, once they are inside the human body, fast neutrons travel freely and widely, creating many recoil protons that generate an extensive collection of ionization tracks throughout the body.
If the radiation source remains external to the body, only gamma radiation and neutrons pose a serious health threat. Alpha and beta radiation do not penetrate far enough through the skin to be considered very dangerous from an external radiation hazard perspective. If alpha or beta radiation sources somehow get deposited inside the body, however, they represent an internal radiation hazard that can be very dangerous.
This brief discussion shows that trying to measure how ionizing radiation affects the human body (or other living systems) is a tricky and complicated task. There are many factors that go into the calculation, including the type of radiation emitted by the source, the strength of the source, and the circumstances of the exposure. Health physicists recognize that the most important thing to know is how much energy carried by the ionizing radiation is actually deposited in a person’s body. They need this information because biological damage increases with the amount of energy absorbed by cell tissue due to ionizing radiation. Quite logically, health physicists have selected absorbed energy as the basis for several fundamental radiation protection quantities called dose.
To calculate the size of the dose, health physicists must first know the amount of energy emanating from the nuclear radiation source. The amount of energy depends on two factors: the activity of the source, that is, the number of radiation particles being emitted each second, and the energy per particle (equation 4.13 provides a definition of activity). The product of these two factors is the power of the source, or the total amount of energy being emitted per second.
Nuclear Binding Energy
