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# Decay Constant

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1. 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?
2. It's defined in the equation

where A is the activity and N the number of nuclei.
This is also written as

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.
3. 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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