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 half-life 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 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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