MadSci Network: Physics |

Posted By:

Area of science:

ID:

The half-life for decay of a sample of radioactive material is described, even in college textbooks, as being the time it takes half the radioactive atoms in a sample to decay. See for instance http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/halfli.html . That definition does not apply to the decay of small numbers of atoms. For instance, if a sample contains 10 atoms with a half-life of 1 minute, it is not certain that precisely 5 atoms will remain after a minute passes. Why not? There is a probability that a radioactive atom will decay. If you chose that atom's half-life as your unit of time, the probability of decay could be represented as 0.5 decays per half-life. If you had 10 such atoms, however, and counted them again after one half-life had passed, the number of decayed atoms could be represented by any number from 10 to 0. Why? Your question, though, was about 1 atom. If you could really monitor a single specific atom, however, it might never be observed to decay. To clarify these comments, try the following experiment intended to illustrate radioactive decay properties of a single atom. Take a single coin and flip it. The probability the coin will land with its 'head' side showing is equal to the probability the coin will land with its 'tail' showing. The probability that something, heads or tails, will happen when you flip the coin is 1.0. Since there are only two equal probabilities, the probability of heads is 0.5 and the probability of tails is 0.5. You can simulate the decay of a single radioactive atom by assuming each coin flip represents the passage of one half-life. Let heads represent no decay and tails represent decay. Where probabilities are concerned, the outcome of your first coin flip would not be very helpful. If you stopped after the first flip, you might conclude that a single atom will decay in a time less than a half- life, as the probability of a decay (tails) is 0.5. To address this difficulty, set up a series of bins labelled something like 'Decayed after 1 flip', 'Decayed after 2 flips', etc. You could accomplish the same result by setting up a two column table with one column labelled Number of Flips and the second column labelled Decays. Now, you flip your coin until the first occurrence of tails and place the coin in the appropriate bin or make a tick mark in the appropriate row of your table. Try doing the experiment 10 times, depict your results on a Decays vs Number of Half-lifes graph. Clear your bins or start a new table and do the flip-to-decay sequence 100 times, then depict the result on the same graph. Examine your graph. If the difference between a small number of atoms and a large number of atoms is still not evident, and you have the time, try again doing the flip-to-decay sequence 1000 times. What should be evident in the graph is the definition of half- life better represents the decay of samples with larger numbers of atoms. You'll also note that some coins took a larger number of flips to decay, maybe you'd even have one or more decays occurring after more than 10 flips. There's more of interest in such a graph, but that would further complicate this answer. To summarize, radioactive decay of an atom is statistical rather than deterministic, you cannot state precisely when an atom will decay. If there are a large number of atoms in a sample, however, the half-life can be used to effect a very accurate estimate of the amount of radioactivity that remains after a given time. Additional references that may be helpful in understanding radioactive half-life would be any textbook on nuclear engineering. Familiarity with the first several chapters of a statistics and probability textbook would also help. Thanks for your question. sid

Try the links in the MadSci Library for more information on Physics.

MadSci Network, webadmin@www.madsci.org

© 1995-2003. All rights reserved.