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Radiation Exposure and Contamination
Measurement of radiation
Conventional units of measurement include the roentgen, rad, and rem. The roentgen (R) is a unit of exposure measuring the ionizing ability of x-rays or gamma radiation in air. The radiation absorbed dose (rad) is the amount of that radiation energy absorbed per unit of mass. Because biologic damage per rad varies with radiation type (eg, it is higher for neutrons than for x-rays or gamma radiation), the dose in rad is corrected by a radiation type quality factor; the resultingequivalent doseunit is the roentgen equivalent in man (rem). Outside the US and in the scientific literature, SI (International System) units are used, in which the rad is replaced by the gray (Gy) and the rem by the sievert (Sv); 1 Gy=100 rad and 1 Sv=100 rem. The rad and rem (and hence Gy and Sv) are essentially equal (ie, the quality factor equals 1) when describing x-rays or gamma or beta radiation. The amount (quantity) of radioactivity is expressed in terms of the number of nuclear disintegrations (transformations) per second. The becquerel (Bq) is the SI unit of radioactivity; one Bq is 1 disintegration per second (dps). The conventional unit, Curie (Ci), is sometimes still used in the US, where one curie is 37 billion Bq. This is equivalent to 37,000 megabecquerels (MBq) or 37 gigabecquerels (GBq). 