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A question about radioactivity.

  1. Oct 11, 2013 #1
    Hi,

    A radioactive substance which has a half-life of, say 5 years, will 'certainly' decay to half its amount after 5 years ( i am good at maths!).

    My question is,

    When radioactivity is a random process, where does this 'certainty' come into picture?


    Thanks.
     
  2. jcsd
  3. Oct 11, 2013 #2

    phinds

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    radioactivity is a random process at the micro level but it is subject to statistical distributions that accumulate at the macro level.

    So all that can be said about a single atom that decays is that it will decay sometime during the life of the universe and that there's a 50:50 chance that the decay will occur within the half-life period.

    A kilogram of such material has so many atoms that statistically you can be sure that very close to half of them will have decayed by the end of the half-life period.
     
  4. Oct 11, 2013 #3

    UltrafastPED

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    The "random process" follows a rule: it follows the exponential distribution.

    Here is an in-depth mathematical discussion:
    http://depts.washington.edu/amath/courses/423-winter-2010/note2.pdf [Broken]

    As to your question about "certainly", well, this word doesn't belong in this description! And smaller the quantity of material being observed, the less certainty. That is, for large amounts the statistics predict that the observed half-life will be 5 years; but for small quantities the uncertainty becomes greater and greater.

    After all, the exponential decay process has no memory ... it does not know what has happened in the past, nor what will happen in the future. But the probability is there for each and every atom.
     
    Last edited by a moderator: May 6, 2017
  5. Oct 11, 2013 #4

    HallsofIvy

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    "Certainly"= "plus or minus a few atoms"!
     
  6. Oct 11, 2013 #5
    To clarify:

    The random process doesn't necessarily follow exponential distribution. All it follows is the probability of a given atom decaying in a certain time-frame. The exponential distribution is an effect caused by this probability and the fact that atoms are disappearing and will no longer be accounted for.

    Say within time τ there is a probability of p that a given atom will decay then we can effectively conclude that after time τ the number of atoms left relative to the initial count is 1-p. Now because there are fewer atoms then the relative number of atoms left after time xτ is $$(1-p)^x.$$ And this represents the exponential decay process.
     
    Last edited by a moderator: May 6, 2017
  7. Oct 11, 2013 #6

    russ_watters

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    I simply say part is random and part is not. Which will decay next is random but the average rate of decay is not.
     
  8. Oct 11, 2013 #7
  9. Oct 11, 2013 #8

    jtbell

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    Who said/wrote 'certainly'? (If it wasn't you, that is)
     
  10. Oct 21, 2013 #9
    What is the reason for this?

    'Large amount' is a relative term. When does amount become large, when we consider 100000 atoms or 100000000 etc.
     
  11. Oct 21, 2013 #10

    sophiecentaur

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    The Casino will, "certainly" make money on roulette because of the 0 on the wheel. Individuals can still take vast levels of winnings away with them, though. There is no conflict between those two statements. You need to understand what statistics is actually telling you and appreciate that there is variation around a general trend.
     
  12. Oct 21, 2013 #11

    UltrafastPED

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    Look up the Law of Large Numbers ... it is a precise mathematical statement which applies in this situation.
     
  13. Oct 21, 2013 #12

    jtbell

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    If we start out with N undecayed atoms, after one half-life we expect to have N/2 left. The actual number will probably not be exactly N/2 because of the randomness of the process. The statistics are exactly the same as with coin-tossing: the probability is 0.5 that any particular atom has not yet decayed, versus 0.5 for any particular coin coming up "heads."

    It turns out that the standard deviation of the number left after one half-life is ##\sigma = \sqrt{N} / 2##. See for example the section "Statistical Uncertainties" in www.csupomona.edu/~pbsiegel/bio431/texnotes/chapter2.pdf‎ [Broken], in particular equation (21), ##\sigma^2 = Npq##. In this case p = q = 1/2.

    Starting with 100 atoms, after one half-life you expect to have 50 ± 5, i.e. 50 ± 10% left.

    Starting with 1000000 atoms, after one half-life you expect to have 500000 ± 500, i.e. 500000 ± 0.1% left.

    Starting with 1020 atoms (< 0.001 mole), after one half-life you expect to have (5 x 1019) ± (5 x 109), i.e. (5 x 1019) ± 0.00000001% left.
     
    Last edited by a moderator: May 6, 2017
  14. Oct 21, 2013 #13
    To put it another way, let's say you start out with 1000 atoms and the half life is 1 year. That is, there's a 50/50 chance that each atom will have decayed by the 1 year mark. After 1 year, there's still a chance that none of the atoms will have decayed, but the odds of that happening are one in 21000 or about one in 10301, which is ludicrously small. And 1000 isn't even that many atoms; in a 12 g sample of carbon you have around 6e23 atoms.

    If we have a large number of atoms, then after one half-life we could find that any number of those atoms has decayed (anywhere from none to all), but probability tells us that it's extremely unlikely that the number of decayed atoms will be very far from half. Still, it's incorrect to say with 100% certainty that exactly half of the atoms will decay.
     
  15. Oct 21, 2013 #14

    sophiecentaur

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    Likewise, it's impossible to predict the outcome of any experiment with total certainty. That's what Science is like.
     
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