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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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