Posted by Anonymous on Tuesday, May 5, 2009 at 10:06am.
Suppose 32000 radioactive nuclei are in a sample. About how many remain after two days if the halflife is 22 hrs? What is the initial activity of the sample in decays per minute?

physics  drwls, Tuesday, May 5, 2009 at 11:24am
Two days is 48 hours or 2.18 halflives. 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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