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Homework Help: Nuclear Physics decay

  1. Oct 8, 2011 #1
    Problem is:

    have a mother and daughter sample, [tex]A[/tex] and [tex]B[/tex] respectively. both are radioactive. The number of daughter nuclei at time t is given by (*):

    [tex]n(t)=\frac{N_{0}\lambda_{A}}{\lambda_{B}-\lambda_{A}}[e^{-\lambda_{A}t}-e^{-\lambda_{B}t}][/tex]

    where N_0 is number of mother nuclei at t=0 and n(t) is number of daughter nuclei at time t.

    A has [tex]\tau_{\frac{1}{2}}=23minutes[/tex] and B has [tex]\tau_{\frac{1}{2}}=23days[/tex].

    A is beta only emitter. B emits gamma and Beta. If A has been made and purified and 11.5minutes after this, the sample emits 1000 gammas/second and some time later the sample again emits 1000 gammas/second - how much time has elapsed?

    My Thoughts:
    I don't know how to handle this problem for the following reason: To find the rate of gammas being emitter, simply differentiate the equation I showed above (*) to get:

    [tex]\frac{dn}{dt}=\frac{N_{0}\lambda_{A}}{\lambda_{B}-\lambda_{A}}[-\lambda_{A}e^{-\lambda_{A}t}+\lambda_{B}e^{-\lambda_{B}t}][/tex]

    (note can find the lambda_A and lambda_B as we know the half-llife and can also solve (*) for N_0).

    The time dependance of the above equation is a negative exponential... so to solve for t, do some rearranging, and take the natural log... but this will be a linear equation... ie. there will only be one value of t for any dn/dt so it is not clear to me how at 11.5mins can have 1000 gammas/second and then again some time later can also have 1000 gammas/second.

    What to do?
     
    Last edited: Oct 8, 2011
  2. jcsd
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