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 What is "dose"? What units are used to express radiation dos 
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What is "dose"? What units are used to express radiation dose?

"Dose" is a word that in some general English uses and in medical terminology may mean something different than is meant in radiation protection. We talk about taking a "dose" of whiskey or getting our daily "dose" of news or anything else we may like; similarly, in medical applications we get "doses" of medicine. In radiation protection, "dose" has a more specific meaning—it is the energy of ionizing radiation absorbed per unit mass of any material. Mostly we talk about the dose to people, or to parts of the body, but we can define the dose in air, water, human tissue, or any other material. Energy is most often given in units of ergs (erg), joules (J), electron volts (eV), or multiples thereof (for example, kilojoules [kJ] and megaelectron volts [MeV]). Mass is most often expressed in terms of grams or kilograms (g or kg). Special units exist for dose, including the "rad," which is defined as 100 erg/g, and the "gray" (Gy), which is defined as absorption of 1 J/kg. 1 gray is equal to 100 rad. These units similarly are used with multipliers, for example, millirad (mrad = 0.001 rad, one one-thousandth of a rad), milligray (mGy), and microgray (μGy = 0.000001 Gy, one one-millionth of a gray).

In many situations, the energy of radiation absorbed per unit mass of material can be related directly to radiation effects. If a large population of people is exposed to 5.0 Gy (500 rad) all at once, about half may be expected to die within 60 days of this exposure. If the skin is exposed to more than a few Gy of radiation (a few hundred rad), this may cause some transient or permanent reddening and, at higher doses, permanent and more severe damage may occur. We are all exposed to around 3 mGy (300 mrad) of radiation every year from natural sources coming from space and radioactive sources in the earth, building materials, and other natural sources. In some situations, however, we find that the absorbed dose does not tell the whole story. Some types of radiation in certain experimental conditions cause more observed effects, given the same amount of absorbed dose, than others. Factors, called "radiation weighting factors" (earlier called "quality factors"), are used to convert absorbed doses (in rad or gray) to "equivalent doses." These equivalent doses have different names, the rem and sievert (Sv). As with dose, 1 Sv = 100 rem, and multipliers are employed (for example millirem [mrem] and millisievert [mSv]). Equivalent dose is only defined for human tissue (that is, not for air, water, etc.).

Another quantity sometimes used in radiation protection is the "effective dose." If the whole body is exposed to radiation more or less uniformly, we can define a single number that gives the dose or equivalent dose to any organ and the whole body. If the body is exposed in a nonuniform manner, however, it becomes more difficult to compare different exposures. Different x-ray procedures expose different organs to different doses, and different radioactive materials inside the body tend to concentrate in different organs, giving a different pattern of dose. Using values that approximately represent the likelihood that the different organs may express radiation effects, organ weighting factors were developed. If each organ is multiplied by its weighting factor and the values are added up, we obtain a dose that is "effectively" like a uniform whole-body dose. We can then compare different nonuniform exposures or add them together to express the total risk of a mixed-exposure situation (for example, a uniform whole-body exposure to an external source in addition to inhalation of some radioactive iodine).
Health Physics Society

Sun Mar 10, 2013 12:02 pm
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