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I was told that a use of both alpha and beta decay is in radiometric dating.

Why is radiometric dating not also considered to be a use of gamma radiation?

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  • $\begingroup$ Atoms don't change isotope or element via the release of electromagnetic radiation. $\endgroup$
    – lemon
    Sep 17, 2017 at 14:32
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    $\begingroup$ I think you were told incorrectly. AFAIK (I can't claim expertise), radiometric dating doesn't use radiation, it measures the decay products left after the radiation happened, which could have been a billion years ago :-) $\endgroup$
    – jamesqf
    Sep 17, 2017 at 17:16
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    $\begingroup$ Counter example: potassium-argon dating is based on an electron-capture reaction. $\endgroup$ Jun 6, 2018 at 20:08

2 Answers 2

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Radiometric dating tends to use a nucleus that changes into some other easily distinguishable nucleus. For example uranium decays to lead 206 and 207, which can be easily measured in a mass spectrometer. We measure both the uranium concentration and the lead concentration and infer the age from how much of the uranium has changed into lead.

The problem with gamma radiation is it doesn't produce a chemically distinguishable product. Gamma decay is effectively a decay of the excited state of a nucleus to a lower energy state of the same nucleus. So there is no way to tell how much of the original parent nucleide has decayed.

By contrast alpha decay produces a daughter atom with an atomic number lower than the parent by two, and beta decay produces a daughter atom with an atomic number higher than the parent by one. In both cases a mass spectrometer can easily tell the difference between the original atom and the product.

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John's answer is only part of it. There are gamma emitters that aren't simply due to an excited nucleus. However, the ones that occur naturally are part of the uranium decay sequence. They are not at either end of the chain and thus measuring the quantity relative to their parents or daughters only tells you the ratio of the half lives, almost nothing about the age of the item.

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  • $\begingroup$ What nuclide is a gamma emitter from the ground state? Your second sentence doesn't make sense. All natural gamma emitters do so from an excited state. $\endgroup$
    – Bill N
    Sep 17, 2017 at 22:26
  • $\begingroup$ en.wikipedia.org/wiki/Commonly_used_gamma-emitting_isotopes $\endgroup$ Sep 17, 2017 at 22:28
  • $\begingroup$ @LorenPechtel, everything I looked at on that page was either an excited form, or decayed by some other mechanism while releasing a gamma ray as a side effect. $\endgroup$
    – Mark
    Sep 17, 2017 at 23:02
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    $\begingroup$ @Mark A gamma is a gamma even if there is also something else going on. $\endgroup$ Sep 17, 2017 at 23:25
  • $\begingroup$ @LorenPechtel said: "There are gamma emitters that aren't simply due to an excited nucleus." What are they? Are you thinking about annihilation photons caused by positron-electron annihilation? Those are external to the nucleus. Yes, a collection of positron emitter atoms will have annihilation photons associated with them. One might colloquially call them gammas, but they do not come from the nucleus. Not all photons are gammas, and gammas aren't gammas because of their energy value. $\endgroup$
    – Bill N
    Sep 19, 2017 at 17:37

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