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All radioactive material undergoes a phenomenon known as half-life, which is the period of time required for half of the radioactive nuclei to decay. Half-life is also known as exponential decay. For example, imagine you have $1024 today and tomorrow you spent half of what you had leaving you with $512. Then the next day you spent half again leaving you with $128. And so on leaving you with $64 on day 4, up until the final day where you are left with $1. The half-life of your money is therefore 1 day, as it halves every day. The same principle applies to radioactive decay. 

Count rate measures the amount of nuclei decaying, similar to putting you hand in your wallet and counting how much money you have left.