units of radioactivity and radiation dose
The becquerel (Bq) is the derived unit of activity (of a radionuclide) in the SI system of units. The unit is named after the French physicist Henri Becquerel. One becquerel is defined as one disintegration per second. Often, in practice, kBq (= 1,000 Bq) and MBq (= 1,000,000 Bq) are used. The becquerel has largely replaced the formerly used unit, the curie, which equals 3.7 × 1010 Bq.
The curie (Ci) is a unit formerly used to measure the activity of a radioactive substance (see radioactivity). Named after Marie Curie, it is defined as that quantity of an isotope that decays at the rate of 3.7 × 1010 disintegrations per second. The curie has been replaced by an SI unit, the much smaller becquerel.
The gray (Gy) is the derived unit of absorbed dose of ionizing radiation in the SI system of units. One gray (Gy) dose is equivalent to 1 joule of ionizing radiation energy absorbed per kilogram of organ or tissue weight. It is named after the British radiologist L. H. Gray (1905–1965). The rad is the older but still occasionally unit of absorbed dose. 1 Gy = 100 rad.
The rad is an old unit used for expressing absorbed dose of ionizing radiation. One rad (rd) represents the dosage absorbed when 1 gram of matter absorbs 100 ergs of energy. Although the rad has been superseded by the gray (1 Gy = 100 rads) for many applications internationally, it is still extensively used, especially in the US.
An acronym for roentgen equivalent man', the rem is the absorbed dose of ionizing radiation that produces the same biological effect as 1 rad of X-rays or gamma rays. The rem was introduced as a result of the observation that some types of ionizing radiation, such as neutrons, produce a greater biological effect for the equivalent amount of absorbed energy than X-rays or gamma rays. In short, the rem is a measure of the biological effect of irradiation. For X-rays and gamma rays, the rem is equivalent to the rad. For other types of radiation, the number of rems equals the number of rads multiplied by a special factor (called the quality factor or relative biological effectiveness) that depends on the type of radiation involved. The rem has been superseded by the sievert in the SI system of units; 1 rem is equivalent to 0.01 sieverts.
The röntgen (R) is a former unit of radiation exposure named for Wilhelm Röntgen. Its value in the SI system of units is such that exposure to 1 röntgen of X-rays or gamma rays causes 0.000258 coulomb of ionization per kilogram of air. It has been replaced in the SI system by the gray.
The sievert (Sv) is the derived unit of equivalent dose of ionizing radiation in the SI system of units, equal to one joule per kilogram (1Jkg-1). One sievert is the absorbed dose of radiation that produces the same biological effect as 1 gray of X-rays or gamma rays. One sievert is equal to 100 rems. It is named after the Swedish physicist Rolf Sievert (1896–1966).