# Worksheet: Calculating the Activity of a Radioactive Source

In this worksheet, we will practice calculating the activity of a radioactive sample after a given amount of time using the sample's half-life.

Q1:

The initial activity of a sample of radioactive material is 800 Bq. What is the activity of the sample after 3 half-lives have passed?

Q2:

What is the activity of a radioactive sample after six half-lives have passed, as a fraction of its initial activity?

• A
• B
• C
• D
• E

Q3:

The initial activity of a sample of radioactive material is 1,200 Bq. What is the activity of the sample after 1 half-life has passed?

Q4:

The shorter the half-life of a radioactive isotope, the more the activity of a sample decreases. The longer the half-life of a radioactive isotope, the more the activity of the sample decreases.

• Aquickly, slowly
• Bslowly, quickly
• Cquickly, quickly
• Dslowly, slowly

Q5:

Which of the following is the correct definition of the half-life of a radioactive substance?

• AThe half-life of a substance is the number of unstable nuclei within it that have decayed in 10 minutes.
• BThe half-life of a substance is the number of unstable nuclei within it that have decayed in 1 hour.
• CThe half-life of a substance is the amount of time it takes for the number of unstable nuclei to decrease by a quarter.
• DThe half-life of a substance is the amount of time it takes for half the unstable nuclei in the substance to decay.
• EThe half-life of a substance is the amount of time it takes for all of the unstable nuclei within it to decay.

Q6:

Carbon-14 has a half-life of 5,730 years. If a sample of carbon contains 200 carbon-14 atoms, how many carbon-14 atoms did it contain 28,650 years ago?

Q7:

The graph shows the activity of 3 radioactive samples against time.

Which sample has the shortest half-life?

• ASample B
• BSample C
• CSample A

Which sample has the longest half-life?

• ASample C
• BSample A
• CSample B

What is the half-life of sample C?

Q8:

Argon-41 has a half-life of 110 minutes. If a sample of argon contains 2,400 atoms of argon-41, how many atoms of argon-41 are left after 5 hours and 30 minutes?

Q9:

A laboratory technician orders a sample of radioactive material. When it arrives, he measures its activity at 300 Bq. If the time between the sample being produced and it being delivered to the technician was equal to 2 half-lives, what was the activity of the sample when it was produced?

Q10:

What is the remaining percent of the original radioactive atoms in a radioactive sample after 4 half-lives have passed?

• A
• B
• C
• D
• E

Q11:

Elizabeth wants to find out the half-life of a radioactive sample. She measures the activity of the sample over time, and her results are shown in the table. What is the half-life of the sample?

 Time (s) Activity (Bq) 0 10 20 30 40 50 800 673 566 476 400 336

Q12:

A scientist has a pure sample of a radioactive isotope of oxygen. He wants to find out which isotope it is and does so by taking readings of the radioactivity of the sample over time and finding the half-life. He plots the radioactivity of the sample over time, as shown in the graph.

 Isotope Half-Life (seconds) Oxygen-14 Oxygen-15 Oxygen-19 Oxygen-20 Oxygen-21 Oxygen-22 70.6 122 26.5 13.5 3.42 2.25

What is the half-life of the sample?

The table shows the half-lives of different isotopes of oxygen. Which isotope of oxygen does the scientist have?

• AOxygen-21
• BOxygen-20
• COxygen-15
• DOxygen-22
• EOxygen-14