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Decay chain (Bi -> Po -> Pb)

  1. Dec 1, 2009 #1
    Hi I'm trying to do a question on nuclear decay chains. The question is:

    Bi-210 decays to Po-210 by beta decay (half life = 7.2 days), and this decays by alpha decay to Pb-208 (half life = 200 decays). If A substance is initially pure Bi-210, when does the alpha emmision peak?

    So far Ive got
    Bi(0)=1, Po(0)=Pb(0)=o

    Bi(t) = Bi(0)exp(-Lt) (L being the decay constant, t_1/2 = ln(2)/L)

    and dPo(t)/dt = L(Bi(0)exp(-Lt)) -L'Po(t)

    Obviously I just need to find when Po(t) is at a maximum, to find the corresponding maximum in alpha emmision, but I can't solve for Po(t)

    can anyone help?
  2. jcsd
  3. Dec 1, 2009 #2


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    Staff Emeritus
    Science Advisor

    Well, think on mean Bi-210 => to Po-210 => Pb-206. Half-lives are 7.2 and 200 days respectively, although reliable sources indicate half-life of Po-210 is 140 d (Hyperphysics) or 138.376 d (BNL NNDC).

    This is at the bottom end of the U-238 decay series.

    Also - apparently Bi-210 can decay to Tl-206, which beta decays to Pb-206, but it's almost 100% alpha.

    To solve the problem, make sure one has the correct half-lives or decay constants.

    One has to develop two differential equations. One deals with the change (decay) in amount of Bi-210, which is also the production rate of Po-210. The other equation deals with the production and decay of Po-210 (production of Pb-206). The peak alpha emission from Po-210 occurs when the amount of Po-210, or when the rate of change is zero (dP/dt = 0), or when production rate = decay rate.

    This is called transient equilibrium.
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