Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 22

Since radioactive isotopes are used because of their radioactivity, the unit for expressing the quantity of radioactive material is based on the disintegration rate. Thus, for example 3.7 × 10¹⁰ atoms in 1 g of ²²⁶Ra decay in 1 s, while 1.7 × 10¹⁴ atoms in 1 g of ²¹⁰Po decay in 1 s. One gram of ²¹⁰Po is much more radioactive than 1 g of ²²⁶Ra. Clearly, mass is not the most appropriate unit for specifying the quantity of a radioisotope. The traditional unit for the quantity of a radioisotope is Curie, symbolized by Ci. The Ci is defined as that quantity of a radioactive material in which 3.7 × 10¹⁰ atoms are transformed in 1 s. The commonly used subunits are the millicurie, mCi (10⁻³ Ci), microcurie, μCi (10⁻⁶ Ci), nanocurie, nCi (10⁻⁹ Ci), and picocurie, pCi (10⁻¹² Ci).

In the SI system, the unit for the quantity of radioactivity is the becquerel (Bq). One becquerel is defined as that quantity of radioactive material in which one atom disintegrates in 1 s. Since the Bq is such a small quantity of radioactive material, the kilobecquerel (kBq) (10³ Bq), megabecquerel (MBq) (10⁶ Bq), and the terabecquerel (10¹² Bq) are frequently used. 1 MBq = 27 μCi.