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Decay Constant

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    How would I define the decay constant in an exam? I was always told it was a probability thing, but would I be right in saying that the decay constant is how many decays take place per second?
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    It's defined in the equation
    A=\lambda N
    where A is the activity and N the number of nuclei.
    This is also written as
    \displaystyle  -\frac{dN}{dt}=\lambda N

    It's the constant of proportionality in the equation relating the activity of a sample to the number of active nuclei present in it.

    The physical significance of the constant is one of probability.
    It signifies the probability of an active nucleus decaying in a interval of one second.
    Or, the fraction of nuclei that decay in a sample in unit time.
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    The probability of an individual nucleus decaying per second, and it's the constant of proportionality. According to past papers you only get the mark for saying both.

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