Units of Measurement for Radioactivity

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Atomic and Nuclear Physics DOE-HDBK-1019/1-93 RADIOACTIVITY Rev. 0 Page 31 NP-01 The activity (A) of a sample is the rate of decay of that sample.  This rate of decay is usually measured in the number of disintegrations that occur per second.  For a sample containing millions of atoms, the activity is the product of the decay constant and the number of atoms present in the sample. The  relationship  between  the  activity,  number  of  atoms,  and  decay  constant  is  shown  in Equation (1-3). A   = N (1-3) where: A   = Activity of the nuclide (disintegrations/second) = decay constant of the nuclide (second  ) -1 N   = Number of atoms of the nuclide in the sample Since    is a constant, the activity and the number of atoms are always proportional. Units of Measurement for Radioactivity Two common units to measure the activity of a substance are the curie (Ci) and becquerel (Bq). A curie is a unit of measure of the rate of radioactive decay equal to 3.7 x 10    disintegrations 10 per second.  This is approximately equivalent to the number of disintegrations that one gram of radium-226 will undergo in one second.  A becquerel is a more fundamental unit of measure of radioactive decay that is equal to 1 disintegration per second.   Currently, the curie is more widely used in the United States, but usage of the becquerel can be expected to broaden as the metric system slowly comes into wider use.  The conversion between curies and becquerels is shown below. 1 curie = 3.7 x 10    becquerels 10 Variation of Radioactivity Over Time The rate at which a given radionuclide sample decays is stated in Equation (1-3) as being equal to the product of the number of atoms and the decay constant.   From this basic relationship it is possible to use calculus to derive an  expression which can be used to calculate how the number of atoms present will change over time.  The derivation is beyond the scope of this text, but Equation (1-4) is the useful result. (1-4) where: N = number of atoms present at time t N = number of atoms initially present o = decay constant (time  ) -1 t = time


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