Posted by Anonymous on Tuesday, May 5, 2009 at 10:06am.
Two days is 48 hours or 2.18 half-lives. The original number of radioactive nuclei, 2 days earlier, is higher by a factor 2^2.18 = 4.53, so there were N0 = 145,000 radioactive nuclei then.
There is a formula that relates activity per molecule (k) to half life (T). It is k = ln2/T = 0.693/T
T = 22 hours = 1320 min
The initial activity is
A = N0*k = (145,000)(0.693)/1320
= 76 decays/min
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