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I was recently doing some physics tuition on radioactivity and the student claimed her chemistry teacher had said that radioactive substances can be grouped into two divisions: those whose half-life is constant and those whose half-life changes over time.

I had never heard of this before and can't think of any reason why a half-life should change, so does anyone else know anything about this?

(I know some half-lives can be altered under certain conditions, but I'm talking about a natural change over time).

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  • $\begingroup$ Would anyone like to add new thoughts to this question? The first answer says "No; half lives are constant". The second answer says "Yes; minor, unexplained variance has been measured." The third answer says "Half lives, by definition, are incompatible with quantum mechanics" implying, presumbly, that either our modelling of half lives or of quantum mechanics, is invalid. $\endgroup$
    – youcantryreachingme
    Commented Jun 22, 2017 at 22:56

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The short answer is no: halflives are constant.

However, let's discuss a situation in which that comment might have some kind of truth behind it. If you have a parent nucleus that decays to a radioactive daughter so that there will be two (or more) decays before stability. In general there are two possibilities for this:

  • The daughter has a shorter halflife than the parent. In this case the concentration of the daughter is always $\displaystyle\frac{\tau_\text{daughter}}{\tau_\text{parent}}$ of the parent concentration. This means that the concentration of the daughter actually decays on the parent's halflife (because the daughter is constantly refreshed from the parent).
  • The daughter has a longer half life than the parent. In this case the daughter will accumulate steadily as the parent decays away.

The latter case is interesting to us here because at the start the sample will register an activity that decays with the parent's (short) halflife, but after a number of those halflifes have passed the activity of the sample will be dominated by the daughter and exhibit a longer halflife.

That is something that your instructor could have meant which would not be wrong. However, the halflife of each isotope remains the same: it is only the halflife of the sample (which contains more than one isotope) that varies.

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  • $\begingroup$ Thanks, OP here (sorry, couldn't remember password, had to sign in under a different name). I did consider what you mention, but it doesn't seem to fit particularly well with having two distinct categories of radionuclide. I suspect the answer is one or both of a) the student's misremembering what the teacher said and/or b) the teacher talking nonsense. Wouldn't be the first time... $\endgroup$
    – James
    Commented Jan 9, 2013 at 14:48
  • $\begingroup$ @dmckee: in reference to "daughter has a longer half life than the parent": Has this ever been observed? (Excluding those daughters that are "observationally stable", of course.) $\endgroup$
    – pr1268
    Commented Nov 19, 2017 at 20:11
  • $\begingroup$ @pr Often. The rapid decay of radon leads to a build up of lead-210, for instance. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Nov 19, 2017 at 20:43
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There have been reports of annual modulation of radioactive decay rates in certain elements. That is, a change in decay rates that depends (apparently) on the position of the Earth around the sun. Here is a fairly recent example (disclaimer: I can't get behind the paywall at the moment). The effect is very small and at the moment there is no consensus on the cause. It is probably a systematic error in the measurements themselves. A more exotic (and unlikely) possibility is new physics.

Other examples

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  • $\begingroup$ Well if it varies with the square of the distance to the sun it's probably something new. $\endgroup$
    – Joshua
    Commented Nov 7, 2015 at 5:06
  • $\begingroup$ From that link: "Data obtained during the solar flare of 2006 December 13 exhibited a significant dip in the counting rate of 54Mn nearly coincident in time with a solar flare, thus supporting the suggestion of a connection between nuclear decay rates and solar radiation" $\endgroup$
    – youcantryreachingme
    Commented Jun 23, 2017 at 4:07
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The following may have little practical use (at least so far), but it seems very interesting to me that, strictly speaking, the exponential law (and, therefore, constant half-life) is incompatible with quantum mechanics (this is an old result by Khalfin). It is very difficult to observe the deviations from the exponential law though. Some details and a reference to Khalfin's work can be found in Nature vol. 335, p. 298 (22 September 1988).

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