Activity 01
Activity 01 : Radiation and Radiological Disasters. (1) The quantity A for an amount of radionuclide in a given energy state at a given time, defined as: A(t)= dN/dt where dN is the expectation value of the number of spontaneous nuclear transformations from the given energy state in the time interval dt. The rate at which nuclear transformations occur in a radioactive material. The equation is sometimes given as A(t) = - dN/dt, where N is the number of nuclei of the radionuclide, and hence the rate of change of N with time is negative. Numerically, the two forms are identical. The SI unit of activity is the reciprocal second (s-1), termed the becquerel (Bq). Formerly expressed in curie (Ci); activity values may be given in Ci (with the equivalent in Bq in parentheses) if they are being quoted from a reference that uses Ci as the unit. Specific Activity: Of a material, for the purposes of the Transport Regulations, the activity per unit mass of the material in which the radionuclides are essentially uniformly distributed of a radionuclide, the activity per unit mass of that nuclide. of a material, the activity per unit mass or volume of the material in which the radionuclides are essentially uniformly distributed. The distinction in usage between specific activity and activity concentration is controversial. Some regard the terms as synonymous, and may favour one or the other (as above). ISO 921 distinguishes between specific activity as the activity per unit mass and activity concentration as the activity per unit volume. Another common distinction is that specific activity is used (usually as activity per unit mass) with reference to a pure sample of a radionuclide or, less strictly, to cases where a radionuclide is intrinsically present in the material (e.g. carbon-14 in organic materials, uranium-235 in natural uranium), even if the abundance of the radionuclide is artificially changed. In this usage, activity concentration (which may be activity per unit mass or per unit volume) is used for any other situation (e.g. when the activity is in the form of contamination in or on a material). In general, the term activity concentration is more widely applicable, is more self-evident in meaning, and is less likely than specific activity to be confused with unrelated terms (such as 'specified activities'). Activity concentration is therefore preferred to specific activity for general use in safety related IAEA publications. (2) See: Facilities And Activities
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