Unit 4 - Mark Rosengarten

A laboratory sample of 32 P triggers 100. clicks per minute in a Geiger-Mueller counter. How many days ago
did the 32 P to decay enough to produce 1600. clicks per minute?
Find out how many half-lives it takes for the counter to go from 400 to 50 clicks per minute. Cut 1600 in half until you
get to 100:
1600 800 400 200 100
So, you needed to cut 1600 in half FOUR times to get to 100, so 4 half-lives have gone by.
Now, look up the half-life of 32 P on Reference Table N. It’s 14.3 days. That’s how long each half-life is. If four of these
half-lives have gone by:
14.3 days/half-life X 4 half-lives = 57.2 days ago, the counter would have read 1600 clicks per minute.
3) You want to find out how long the half-life is, knowing how much a sample has decayed over a given
amount of time.
Step 1: Determine how many times you can cut your original amount in half in order to get to your final amount. This
is the number of half-lives that have gone by.
Step 2: Divide the time that has elapsed by the number of half-lives that have passed.
A radioactive sample is placed next to a Geiger counter and monitored. In 20.0 hours, the counter’s reading
goes from 500 counts per minute to 125 counts per minute. How long is the half-life?
First, find out how many half-lives it will take for the counter to go from 500 to 125 counts per minute:
500 250 125 You needed to cut 500 in half TWO times to get to 125, so 2 half-lives have gone by.
Half-life = time elapsed / # of half-lives = 20.0 hours / 2 half-lives = 10.0 hours per half-life!
A sample of pure radioactive isotope is left to decay. After 40.0 days, the sample is placed in a mass
spectrometer, and it is determined that the sample only 25% of the original isotope remains. How long is the
half-life?
First, find out how many half-lives it will take for 100% of a sample to decay to 25%:
100 50 25 It takes TWO half-lives for the sample to decay from 100% to 25%.
Half-life = time elapsed / # of half-lives = 40.0 days / 2 half-lives = 20.0 days per half-life!
© 2011, Mark Rosengarten AE 12