There are essentially two types of radiation - ionizing and non-ionizing. When ionizing radiation passes through a medium such as air or body tissue, it may tear electrons loose from some of the atoms in the medium, thus splitting the atom into a negative ion (the electron) and a positive ion (the nucleus and the remainder of the electron shell).
Non-ionizlng radiation, no matter how strong, will not do that; it includes low frequency radiation in the immediate neighborhood of power lines, radio waves, microwaves (used in FM radio, TV, and microwave ovens), infrared, visible, and ultraviolet light. Ionizing radiation includes X-rays and the radiation due to radioactivity.
Both types of radiation, like milk and honey, can be harmful if absorbed in unreasonably large quantities. But unlike milk and honey, there is no getting away from them; there is radioactivity all around us, and has been here long before man appeared on the planet.
Radioactive radiation arises by spontaneous disintegration of the nuclei of radioactive elements such as potassium 40 (contained in human blood) or any isotope of uranium, to name but two elements that occur in pristine nature. The radiation is of three types¾alpha particles (helium nuclei, absorbed in a few inches of air or a sheet of paper); beta particles (electrons, absorbed in a few feet of air); and gamma rays, which are highly penetrating, but can be absorbed by many feet of earth or a few feet of concrete.
Radioactivity is measured in four ways: 1 ) the activity (measured in curies) is the number of disintegrations per second. But not all disintegrations have the same effect; some elements shoot out more, others less energy, and their relative amounts of alpha, beta and gamma radiation also differ. The effect is given by 2) the exposure (measured in rontgens); it is the electric charge of the electrons torn out of their atoms per kilogram of air exposed to the radiation.
Both activity and exposure are characteristic of the radioactivity of the source; they do not tell us what their effect on the exposed medium (other than airs is. This is given by 3) the absorbed dose (measured in rads), the energy absorbed by unit mass of the medium. This energy is proportional to the duration of irradiation, i.e., it increases with time. However, to gauge health effects, the absorbed dose does not give sufficient information, since the human body is not homogenous material like air or metal; the same dose will have different effects on different parts of the body, and even for the same part of the body it will have effects differing by the type of radiation. The absorbed dose is therefore multiplied by a relative biological effectiveness (RBE) factor to take account of these modifications; the result is 4) the dose equivalent (measured in rems, or more often in millirems, i.e. thousandths of one rem).
For those who find all this confusing, we recommend sticking just to the rem, the unit important in judging health effects. On the other hand, the reader longing for more confusion will be happy to know that apart from the rontgen, these widely used units do not conform to the International System ot Units, where the units are the becquerel ( = 2.7 x 10 11 curies), the gray (= 100 rads) and the sievert (= 100 rem).
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Vol. 6, No. 5
Newsletter: Access to Energy Newsletter Archive Volume: Volume 6 Issue/No.: Vol. 6, No. 5 Date: January 01, 1979 04:04 PM Title: The "Idealists"
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