|Workers attempting to repair power lines|
at the Fukushima Daiichi Nuclear Power Plant
(Courtesy Tokyo Electric Power)
A material is radioactive if its nucleus is unstable. The SI (Systeme International) unit for radioactivity is the becquerel (Bq), which refers to one radioactive disintegration per second. An older unit (which is still widely used) is the curie (CI), which is the quantity of a radioactive nuclide in which the number of disintegrations per second is 3.7 x 10^10. Thus, 1 Ci = 3.7 x 10^10.
The amount of radiation energy absorbed per kilogram in a material is called absorbed dose. The SI unit for absorbed dose is the gray (Gy). The older units is the rad (radiation absorbed dose). One gray is equal to 1 joule of energy absorbed per kilogram. The gray and the rad are related: 1 Gy = 100 rad. Dose absorbed throughout a person's entire body is referred to as whole body dose (effective dose).
The most common radiations, x-rays, gamma rays, and beta particles, have about the same effect on tissue. However, heavier radiation particles such as neutrons, protons, and alpha particles do considerably more damage on the average per unit of energy deposited. This effect is known as the relative biological effectiveness (RBE) of the radiation. Thus, heavy particles generally have a higher RBE than x-rays.
To place all types of radiation to tissue on an equivalent basis for radiation protection purposes, a new quanty was needed, called the equivalent dose (particular organ). Its unit is the sievert (Sv). The original unit was the rem, which stands for rad equivalent man.
Again, the two basic radiation protection quantities in use are the equivalent dose (particular organ) and the effective dose (whole body), both reported in sieverts. Since in many radiation protection situations the radiation to a worker or to a member of the public is not uniform but instead is concentrated in a particular organ of the body, such as the lungs, it is desirable to convert a partial-body absorbed dose in grays to a whole-body effective dose in sieverts.
Three fundamental techniques are used to protect people from ionizing radiation. These are the use of (1) time, (2) distance, and 3) shielding. (WSJ, 3/24/2011, "Bluebells and Nuclear Energy, Albert B Reynolds")