Becquerel, definition

The basic unit of radioactivity is the curie, Ci. One curie is the amount of radioactive material that emits particles at a rate of 3.7 X 1010 disintegrations per second (dps), or 2.2 X 1012 min-1 (dpm). Amounts that large are seldom used in experimentation, so subdivisions are convenient. The milli-curie (mCi, 2.2 X 109 min-1) and microcurie (yu,Ci, 2.2 X 106 min-1) are standard units for radioactive measurements (see Table 6.2). The radioactivity unit of the meter-kilogram-seconds (MKS) system is the becquerel (Bq). A becquerel, named in honor of Antoine Becquerel, who studied uranium radiation, represents one disintegration per second. The two systems of measurement are related by the definition 1 curie = 3.70 X 1010 becquerels. Since the becquerel is such a small unit, radioactive units are sometimes reported in MBq (mega, 106) or TBq (tera, 1012). Both unit systems are in common use today, and radioisotopes received through commercial sources are labeled in curies and bequerels. 

It is important to note that in the various studies reviewed in the preparation of this document, dose levels have been presented by those authors in several ways. In order to facilitate comparisons among studies, these levels have generally been converted to an equivalent dose in microcuries (p Ci) and kilo-Becquerels (kBq). The historical definition of one curie is the disintegration rate exhibited by one gram of radium. There are 0.027 pCi per kBq. In this document, comparisons are usually made between total administered amounts of radioactivity, in pCi/kg and kBq/kg, instead of using a daily dosage level. 

In order to understand the effects of radiation on any material, let us define how one measures radiation. Radiation is measured in several units. The SI unit for radioactivity is the becquerel (Bq), which is defined as one disintegration (d) per second (s) (Bq = d/s). However, the most common unit is the curie (Ci) and its definition is based on the number of radium-226 isotopes present in 1 gram of the element that disintegrate in one second. One Ci is thus exactly equal to 3.70 x 1010 disintegrations per second... 

Radioactivity is measured in decays per second, the SI unit of which is the Becquerel (Bq) which is defined as one decay per second. The Curie (Ci), which originally corresponded to the number of decays per second in 1 g of radium, is still used and is now related to the Becquerel by the definition ... 

In 1896 there came the discovery of radioactivity by Henri Becquerel and the discovery of radium by Pierre and Marie Curie. Soon thereafter it was recognized that radioactive changes involve the spontaneous conversion of atoms of one element into those of another. It then became necessary to change the definition of element this was done by saying that one element could not be converted into another by artificial means. 

Radionuclides are, by definition, unstable and decay by one, or more, of the decay modes alpha, beta-minus, beta-plus, electron capture or spontaneous fission. Although strictly speaking a de-excitation rather than a nuclear decay process, we can include isomeric transition in that list from the mathematical point of view. The amount of a radionuclide in a sample is expressed in Becquerels -numerically equal to the rate of disintegration - the number of disintegrations per second. We refer to this amount as the activity of the sample. Because this amount will change with time we must always specify at what time the activity was measured. 

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