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Radioactive Decay Half-Life

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Radioactive Decay Half-Life

A science team is monitoring a sealed sample. Use the half-life model to work out how its activity changes over time. Assume the sample decays by half at each half-life; no new radioactive material is added. A calculator may help.

The model

Radioactive decay is a change in an unstable atomic nucleus that releases radiation. Half-life is the time taken for half the radioactive nuclei in a sample to decay. The half-life is constant for a particular isotope, but individual nuclei decay unpredictably.

1.A sealed sample begins with 640 radioactive nuclei. Its half-life is 3 hours. How many radioactive nuclei are expected to remain after 9 hours? Show the number of half-lives in your working.

2.A detector records 200 counts per minute at the start and 50 counts per minute 12 days later. Assuming the change is due to decay, what is the sample's half-life?

3.A sample has an initial activity of 960 Bq and a half-life of 4 years. Select its expected activity after 12 years.

120 Bq
240 Bq
480 Bq
60 Bq

4.Complete the table for a sample that starts at 80 g and has a half-life of 5 hours. Enter the expected mass remaining at each time.

Time elapsed

0 h

5 h

10 h

15 h

Mass remaining (g)

5.An isotope has a half-life of 8 minutes. What fraction of the original radioactive nuclei remains after 24 minutes?

1/2
1/4
1/8
1/16

6.True or false: After two half-lives, half of the original radioactive nuclei remain.

TrueFalse

7.A student says, “After three half-lives, the sample has completely disappeared.” Explain what is wrong with this statement.

8.A radioactive material is used in a medical tracer. Give one reason knowing its half-life is useful when choosing or using the tracer.

9.Carbon-14 has a half-life of about 5,730 years. A once-living object has about one quarter of the carbon-14 activity of a comparable living sample. Estimate how long ago it stopped exchanging carbon with its environment. State the assumption you make.

10.The half-life of an isotope is 10 years. The sample starts with 400 mg. Calculate the expected mass after 30 years and describe the pattern you used.