Scientists define the absorbed dose as the energy deposited in an organ or mass of tissue per unit mass of irradiated tissue. Two common absorbed dose units are the rad and the gray. The rad is the traditional unit of absorbed dose and corresponds to the deposition of 100 ergs per gram of irradiated tissue. An erg is a unit of energy previously used in physics. It corresponds to the work done by a force of one dyne acting through a distance of one centimeter. The SI unit of absorbed dose is called the gray (Gy) in honor of the British radiobiologist L. H. Gray (1905–1965). An absorbed dose of one gray corresponds to the deposition of one joule of energy (from ionizing radiation) per kilogram of irradiated mass. Since 107 ergs = 1 joule, the rad and the gray are related as follows: 100 rad = 1 gray.
The absorbed dose is an important concept that leads to two closely related concepts, as shown in Figure 4.22. When nuclear energy is deposited primarily in a single organ (such as beta radiation from iodine-131 in the thyroid), health physicists prefer to calculate the actual dose to that particular organ. They express that calculation in terms of the organ dose (in rads or grays). When the nuclear radiation energy is deposited throughout the body (as often happens when a person is exposed to fast neutrons or a widely distributed source of gamma rays), health physicists use the whole body dose.
There is one key factor, however, that the absorbed dose concept does not take into account; different types of directly and indirectly ionizing nuclear radiation leave different ionization tracks in the irradiated tissue. The more densely populated with ion pairs a track is, the more effective the particular type of ionizing radiation in damaging tissue. For example, alpha particles do not have to cross many body cells to deposit all of their kinetic energy through ionization. Their very short range allows the alpha particles to create an enormous number of ion pairs in a very small tissue volume. Therefore, even though one rad (or 0.01 gray) of alpha radiation represents the same amount of energy deposited in living tissue as one rad (or 0.01 gray) of beta or gamma radiation, the alpha radiation causes much more biological damage due to the way it deposits this absorbed dose.
Health physicists account for these differences by using a radiationweighting factor (wR) that represents the effectiveness of each type of nuclear radiation to cause biological damage. Previously, they used a similar comparative concept called the quality factor (Q). Scientists determine radiation-weighting factors by measuring the occurrence of various biological effects for equal absorbed doses of different radiations. After years of careful research, health physicists can now assign specific numerical values to the weighting factor for certain types of ionizing radiation but still cannot reach a general consensus on weighting-factor values for other types of ionizing radiation. The currently accepted weighting-factor values are: alpha particles = 20, beta particles (electrons and positrons) = 1, gamma rays and X-rays = 1, protons = 2 to 10 (depending on energy), fast neutrons = 20, slow neutrons = 5 to 10 (depending on energy), and fission products and other heavy nuclei = 20.
The radiation-weighting factor is a dimensionless quantity that allows health physicists to compare the biological consequences of equal absorbed doses of nuclear radiation. Equation 4.35 presents one of the basic relationships in health physics and radiation protection. It states that the doseequivalent (symbol: H) is equal to the product absorbed dose (D) and the radiation-weighting factor (wR).
Measuring Radiation Doses in Living Tissue
