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Specific activity is the activity per quantity of a radionuclide and is a physical property of that radionuclide. Activity is a quantity related to radioactivity. The SI unit of activity is the becquerel (Bq), equal to one reciprocal second. Since the probability of radioactive decay for a given radionuclide is a fixed physical quantity (with some slight exceptions, see Changing decay rates), the number of decays that occur in a given time of a specific number of atoms of that radionuclide is also a fixed physical quantity (if there are large enough numbers of atoms to ignore statistical fluctuations). Thus, specific activity is defined as the activity per quantity of atoms of a particular radionuclide. It is usually given in units of Bq/g, but another commonly used unit of activity is the curie (Ci) allowing the definition of specific activity in Ci/g. == Half-life == Experimentally-measured specific activity can be used to calculate the half-life of a radionuclide. Half-life (T1/2) is defined as the length of time for half of a given quantity of radioactive atoms to undergo radioactive decay: Or more generally: Starting with ''N0'', atoms of an element, the number of atoms, ''N'', remaining after time, ''t'', is given by: : The natural log of both sides : The derivative with respect to time, ''t'' : Multiplying both sides by ''N'' : Yields : ''dN''/''dt'' represents the decay rate of atoms. The negative sign shows that the rate is negative, so the number of atoms is decreasing with time. Rearranging terms: : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「specific activity」の詳細全文を読む スポンサード リンク
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