Half-life: Quiz questions



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?



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.