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Where does Pb-204 Come from?

  1. Mar 2, 2010 #1
    I always thought I'd do well on that game show: Are you smarter than a 5th Grader? Guess I'd lose - my educational level stops at the fourth.

    IAE, my son is lead (i.e. Pb) for his 5th grade science project. He has to tell the class where he came from. I had always thought lead came from one of three decay series: Uranium, Thorium, and Actinium (I got that for him easy enough). But that only explains lead 206, 207, and 208. Apparently "natural" lead is isotope Pb-204, and that is the most abundant (I think). But how is that made? I checked my sources and apparently it comes about via the s-process vice the r-process, but no indication of how. I thought s-process nucleosynthesis was only inside stellar cores and stopped at Fe - 56 protons away! All else either comes from explosive nucleosynthesis (which I understand is the r process) or radioactive decay. So where does all the Pb-204 come from then? Am I missing a radioactive series? Or is there more to explosive nucleosynthesis?

    I'm probably going way too far on this for a 5th grade class, but it now has my attention and I can't shake it! It's a helluva day when 5th graders are studying nucleosynthesis, but simply saying that the elements are cooked inside stars or exploding stars or come from radioactive decay is probably sufficient for them to understand.

    TIA!

    Sterling
     
  2. jcsd
  3. Mar 2, 2010 #2

    mgb_phys

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    You can make heavy elements in red giants by s-process, the neutrons from the core create heavy nuclei in the outer atmosphere.
     
  4. Mar 2, 2010 #3
    82 + 2 = 84. 204 + 4 = 208. Check out polonium-208. It has a half-life of 2.9 years. It decays by alpha decay. See

    http://ie.lbl.gov/toi/nuclide.asp?iZA=840208

    Bob S
     
    Last edited by a moderator: Apr 24, 2017
  5. Mar 2, 2010 #4

    Astronuc

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    Pb-204 is not 'natural' lead. The isotopic abundances are:

    Pb-204 1.4%
    Pb-206 24.1%
    Pb-207 22.1%
    Pb-208 52.4%

    Ref: http://www.nndc.bnl.gov/chart/reCenter.jsp?z=82&n=124 (click on Zoom 1)

    So natural lead on earth would have that isotopic composition. In addition to alpha decay of Po-208 as BobS indicated, Pb-204 would arise from electron capture in Bi-204. Bi-204 could come from alpha decay of At-208, but At-208 is more like to decay by electron capture to Po-208, which could also be produced by alpha decay of Rn-212.

    Pb-204 could also be formed by an (n,p) reaction which would have the same effect as electron capture, namely transforming Bi-204 into Pb-204.
     
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