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ACTIVITY in Half Lives, Radioactive Isotopes. Last updated February 17, 2025.

Summary

In this activity, students will learn how scientists study Earth’s atmosphere and climate history through the analysis of ice cores obtained from glaciers. They will then analyze radioisotope data from ice core samples to determine the age of the samples based on the decay of radioactive Kr-81 isotopes. Students will also research other isotopes that can be used to determine ages of different types of samples.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

  • HS-ESS3-6: Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.
  • HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Developing and Using Models
    • Engaging in Argument from Evidence
    • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this activity, students should be able to:

  • Identify ways scientists can use ice cores from glaciers to study Earth’s past.
  • Determine the age of an ice core sample using Kr-81 radioisotope data.
  • Explain why different radioisotopes might be used to determine the age of different samples.

Chemistry Topics

This activity supports students’ understanding of:

  • Nuclear Chemistry
  • Radioactive decay
  • Radiometric dating
  • Half-life

Time

Teacher Preparation: 10 minutes
Lesson: 30-45 minutes

Materials

  • Student Handout
  • Device with internet access for article access in Part 1

Teacher Notes

  • This activity was created to support the celebration of Chemists Celebrate Earth Week (CCEW) and the 2025 theme “Glaciers: Hot Topic, Cool Chemistry!”
  • This activity is a modification and expansion of the optional student activity included in the Radioactive Decay and Peat Bogs activity in the AACT classroom resource library. The Peat Bogs activity, as well as the related Radioactive Decay and Seafloor Data activity, can be used with this activity to help students understand the role of radioactive isotope analysis of various geological samples in studying Earth’s past atmospheric and environmental conditions. The ice core data for this activity is a simplified sample from this research article.
  • Students should have a basic understanding of radioactive decay and half-lives prior to completing this activity. Activities from the AACT library that could help introduce students to these concepts include:
  • Part 1 can be done as homework if class time is limited. It would be beneficial to have students complete Part 2 in small groups so they can interpret the graphs together and discuss their reasoning for the analysis questions.
  • The radioisotope of focus in this activity is Kr-81, which undergoes electron capture. This is not addressed in the student activity, as electron capture is not covered in high school classes as often as alpha, beta, and gamma decay, but you could add a question about the type of radioactive decay if you want students to investigate this topic further.
  • This activity uses a pre-made graph of the nuclear decay of the isotope in question, as it was originally created for a 10th-grade, introductory chemistry course. If this activity were used in a more advanced course, you may choose to remove the graphs and ask students to use a more mathematical pathway to determine the age of a sample. Students could use the equation Age= -(t1/2/0.693) ln⁡(N) where t1/2 is the half-life of the isotope and N is the portion of the original sample remaining, written as a decimal. For example, if a sample of Kr-81 (t1/2 = 229,000 Yr) is 75% of its original concentration, then Age=-(229,000Yr/0.693) ln⁡(0.75)≈ 95,000 years old.
  • If time allows, use the Radioactive Dating Game from PHeT to deepen students’ understanding of how/why various isotopes are used in radiometric dating. This simulation examines the decay of radioisotopes C-14 (in formerly living things) and U-238 (in non-living samples such as rocks). Students can use the graphs to make age predictions of the different objects. This can be done as a whole-class activity, projecting the simulation on the screen, or in small groups. The simulation brings up some interesting discussions points:
    • For non-living objects (rocks), there is no usable C-14 data. So, this technique only works for formerly living things. Similarly, living organisms don’t have measurable levels of U-238 in them, so that isotope is more useful for rocks.
    • The living objects at the surface (the live trees) have no decay, as they are constantly exchanging carbon atoms through eating, breathing, etc. and maintain a stable level of C-14 in their bodies through this exchange. This allows for discussion on the nature of C-14 in living organism and that an observable change in that amount only occurs after it dies.
    • For formerly living things that are really old, the amount of C-14 approaches zero. This allows students to see that the range of time we can explore with C-14 is limited by the half-life of the isotope, which should align with the conclusions they drew in the analysis questions about the use of Kr-81 vs. C-14 in dating glacier samples.
    • The simulation will accept a range of suggested ages as correct, which indicates that some degree of uncertainty is to be expected in the results of radiometric dating (especially over large time scales, such as hundreds of thousands of years or more).

For the Student

Part 1: What Do Ice Cores Reveal About the Past?

Glaciers are like an atmospheric time capsule, preserving information about Earth’s past climate and environmental conditions. Use the following article to learn more about what information scientists can learn from glacial ice cores and answer the questions below:

https://nsidc.org/learn/ask-scientist/core-climate-history

  1. At what temperature are glacier ice core samples stored?
  2. List five types of materials trapped in glacier ice that can inform scientists about past atmospheric conditions.
  3. What can scientists learn by studying oxygen isotopes in ice cores?
  4. What are two things scientists have learned about the Earth’s atmosphere by studying carbon in ice cores?
    1. What evidence do ice cores provide to indicate that the current high levels of CO2 in the atmosphere are a result of human activity?
  5. List three other geological materials (besides ice cores) paleoclimatologists use to study ancient climate conditions.

Research the following questions and cite your sources. (Be sure to use the most recent reliable resources you can find, as scientists are always collecting new samples!)

  1. What is the oldest continuous ice core scientists have recovered? How deep did the ice core go?
  2. What is the oldest glacial ice ever studied? Where did it come from?

Part 2: Dating Ice Cores with Kr-81 Decay

Background

In studying snow, ice, and glaciers scientists often take ice core samples. They drill down into the ice and remove a long cylinder of ice, often many meters long. When ice forms, it traps gases from the air within its structure, preserving a record of the atmosphere. Scientists study the gases trapped at different depths in the ice to try and understand our atmosphere’s past.

In the upper atmosphere, cosmic radiation constantly bombards the atmosphere. When those rays strike atoms, they can trigger a nuclear rearrangement that creates the isotope Kr-81. The amount of cosmic radiation entering the upper atmosphere is fairly constant over time, so the amount of Kr-81 created in the atmosphere is fairly constant over time as well. Once a pocket of air containing Kr-81 gets trapped in glacial ice, the concentration of Kr-81 declines as it undergoes radioactive decay since it is cut off from the atmosphere and new Kr-81 atoms created by cosmic radiation cannot enter the air pocket to replenish those that decay.

In 2014, a group of scientists obtained ice cores from the Taylor glacier in Antarctica and specifically studied the isotope Kr-81 trapped within the ice core. Table 1 is a summary of some of the data they collected from those ice core samples. The table describes the depth within the ice core where the sample came from and the observed amount of Kr-81 in that sample relative to the amount we find in today’s atmosphere.

Table 1: Kr-81 in Ice Core Samples
Depth of Ice Core Sample Percent of Kr-81 relative to Modern background
77.0m 96.0%
309.5m 85.5%
499.0m 78.5%
780.0m 69.0%


The graphs below represent the decay of Kr-81 over time. Graph 1 represents that decay over the course of 800,000 years, while Graph 2 considers only the first 150,000 years.

Analysis

  1. Consider Graph 1 on the previous page.
    1. What is the half-life of Kr-81, according to the graph? Explain how you know.
    2. Approximately how many half-lives of Kr-81 are represented on the graph? Explain how you know.
  2. Use Graph 2 to estimate the age of each ice core sample from Table 1. Record your estimated ages on the table.
    Table 1: Kr-81 in Ice Core Samples
    Depth of Ice Core Sample Percent of Kr-81 relative to Modern background Estimated Age (Years)
    77.0m 96.0%
    309.5m 85.5%
    499.0m 78.5%
    780.0m 69.0%
  1. The following claim is made regarding the findings of this experiment:
    “The findings suggest that the glacier is formed in layers, from the top down. The deepest layers of the glacier are the oldest and have been compressed over time by successive layers forming on top of the glacier.”
    Can the evidence support this claim? If so, cite specific evidence that supports the claim and explain how it supports the claim. If not, cite specific evidence that refutes the claim and explain why it refutes the claim.
  2. After how many half-lives (to the nearest whole number half-life) would the level of Kr-81 be less than 1% compared to modern levels? Less than 0.1%? In each case, how many years would have passed?

Less than 1%: Half-lives ______ Years ____________

Less than 0.1%: Half-lives ______ Years ____________

  1. Perhaps a more familiar form of radioactive dating is carbon-14 (C-14) dating. C-14 has a half-life of 5,730 years. After how many half-lives (to the nearest whole number half-life) would the level of C-14 be less than 1% compared to modern levels? Less than 0.1%? In each case, how many years would have passed?

Less than 1%: Half-lives ______ Years ____________

Less than 0.1%: Half-lives ______ Years ____________

  1. Based on your answers to the previous two questions, suggest a reason why scientists chose to use Kr-81 measurements rather than C-14 measurements for dating these samples.
  2. Using radioactive isotopes to determine the age of geological samples is called radiometric dating. Research other radioisotopes (besides Kr-81 and C-14) that scientists use for radiometric dating. List at least two of them and what types of geological samples they are used to date. (Be sure to use and cite reliable sources!)