A complete understanding of the biological effects of high-energy electromagnetic radiation and corpuscular radiation is not currently available. In the face of this, scientists have devised a system for measuring the amount of energy that is transferred from radiation to an object as well as for estimating the relative damage that a particular kind of radiation can cause. The units that are used currently for sorting out the differences are the gray and the sievert. The gray and the sievert appear to be identical units because they are both expressed in terms of joules of radiation energy per kilogram of object. It is important to keep in mind their difference, however: the gray describes a measurable property of radiation while the sievert describes an estimated property.
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The gray describes an objective property of radiation that can be expressed in simple terms: the amount of energy transferred by radiation to an object. An absorbed dose of one gray is equal to the absorption of one joule of radiation energy by one kilogram of matter. For example, the average person absorbs about 450 micrograys of cosmic radiation in the course of a year. The gray was adopted internationally as a unit of absorbed dose in 1976. Prior to the gray, there was the rad, short for r adiation a bsorbed d ose. The difference between the rad and the gray is a proportionality factor: 100 rads equals one gray. Finally, prior to either of these units, there was another measure of radiation absorption called the roentgen. Although the roentgen describes a different property from energy absorbed per unit mass, the effect of one roentgen on dry air is roughly equal to the rad. The roentgen is defined as the amount of x-ray or gamma ray radiation (electromagnetic radiation) that produces 1/3 x 10-9 coulomb of electric charge in one cubic centimeter of dry air at standard conditions.
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A measurement of “energy absorbed per unit mass” does not tell the entire story, however. Consider the difference between radiation composed of electromagnetic energy and radiation composed of charged particles. It is known that for a fixed amount of energy that is absorbed, the amount of biological cell damage is greater when the radiation is composed of charged particles than when it is composed of electromagnetic energy only. For example, beta rays are streams of electrons and alpha rays are streams of helium nuclei. Both of these types of radiation cause more biological damage per energy absorbed than x-rays and gamma rays, which are forms of high-energy electromagnetic radiation. An estimate of this damage is embodied in the unit sievert, which measures the radioactive dose equivalent. One sievert is equal to one gray multiplied by a relative biological effective factor, Q, and a factor that takes into account the distribution of the radiation energy, N. Specifically, if E represents the radioactive dose equivalent in sieverts, and D is the absorbed dose in grays, then E = QND. The factor Q varies from 1 for electromagnetic radiation to 20 for radiation consisting of high-energy charged particles. Suppose that the distribution of energy of cosmic radiation is identical for both charged particles and electromagnetic energy and is equal to one: N = 1. If the 450 micrograys of absorbed cosmic radiation consist solely of gamma rays (high energy electromagnetic radiation), then Q = 1, and the average person absorbs 450 microsieverts of radiation in one year. Alternatively, suppose that the 450 micrograys of absorbed cosmic radiation consists solely of alpha particles (helium nuclei) with energies of 10 million electron volts. In this case, Q = 20, and the average person absorbs 9,000 microsieverts of radiation in one year.
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The sievert is the correct unit to use when you wish to monitor the biological danger of radiation. The gray is the correct unit to use when you wish to monitor energy absorbed per unit mass. Prior to the sievert, the unit used to monitor the biological effectiveness of radiation was called the rem, short for r oentgen e quivalent m an. Similar to the difference between the rad and the gray, the difference between the rem and the sievert is a proportionality factor: 100 rems equal one sievert.
