Half Life Questions
1. The activity of a radioisotope is 640cpm (counts per minute). Two hours later it has fallen to 4cpm (counts per minute) Find the half life of the sample
ANSWER after one after two after three after four
INITIAL COUNT ( /2 ) half life (/2) half lives (/2) half lives (/2) half lives
640 320 160 80 40
It takes four half lives for the activity to fall from 640 to 40. Hence two hours represents four half lives, so the half-life is 30 minutes.
You can measure the half-life of a radioactive substance using a G-M tube and a counter.
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