Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93RADIOACTIVITYThe basic features of decay of a radionuclide sample are shown by the graph in Figure 10.Figure 10 Radioactive Decay as a Function of Time in Units of Half-LifeAssuming an initial number of atoms No, the population, and consequently, the activity may benoted to decrease by one-half of this value in a time of one half-life. Additional decreases occurso that whenever one half-life elapses, the number of atoms drops to one-half of what its valuewas at the beginning of that time interval. After five half-lives have elapsed, only 1/32, or3.1%, of the original number of atoms remains. After seven half-lives, only 1/128, or 0.78%,of the atoms remains. The number of atoms existing after 5 to 7 half-lives can usually beassumed to be negligible. The Chemistry Fundamentals Handbook contains additionalinformation on calculating the number of atoms contained within a sample.Rev. 0Page 33NP-01
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