Question 3:

     A researcher measures 200 counts per minute coming from a radioactive source at midday.
     At 3 o'clock, she finds that this has dropped to 25 counts per minute.
     What is the half-life of the radioactive source?

Answer:
   The half-life is 1 hour.

After 3 hours, the count rate has fallen from 200 to 25 counts per minute - in other words, it's fallen to one eighth of where it started. Count on your fingers:-    -first finger="half",    -second finger="quarter",    -third finger="eighth" so one eighth of the count rate means that three half lives have passed. If 3 half-lives are 3 hours, then the half-life is one hour.

 

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Created by Andy Darvill, www.darvill.clara.net,Science teacher at Broadoak Community School, Weston-super-Mare, England