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Radioactive Decay
Administration Procedures
Contributed by: Council of Chief State School Officers (CCSSO)


The objective of the event is to help students understand radioactive decay and be able to relate it to the concept of half-life.

The students are given 100 green beads that represent radioactive atoms and 100 white beads that represent stable, non-radioactive daughter atoms. The green beads are placed in one cup and the white beads in another. They are asked to do two trials, one sampling 8 beads at a time and the other sampling 4 beads at a time. The number of green beads removed in each sample should be recorded and replaced with an equal number of white beads. Any white beads removed in a sampling event should be returned to the cup that originally began with all green beads. Once 50 green beads are removed from the cup the elapsed time should be recorded.

Students graph each of the samples to show the relationship between the number of green beads remaining in the cup on the y-axis and the sample on the x-axis. The graphs represent the decay curves for the samples.

Given some basic information, students are asked to explain how many years they would have to be concerned about the radioactivity of the sample and how they would dispose of the material.

A good response will show the student understands the concept of half-life and can use this concept to determine when their sample will be "safe". From the experiment they may also note that sampling 8 beads at a time will become safer in less time than when they sampled 4 beads at a time because it has a shorter half-life. A method of disposal must include limiting exposure of the radioactive material to the environment.

Students will be working groups (2-3) while running the simulation and collecting the data. Students should work alone while responding to the questions.

This task is designed to take students approximately 40-50 minutes to complete.

General Instructions to the Teacher:

  • Students need to be ready to work as soon as the period begins.
  • Group assignments should be made in advance.
  • The materials should be set out at each lab station, if possible.
  • A central supply area, if needed, should be easily accessible.
  • The supply area should have any supplies necessary for the experiment/simulation.
  • All supplies should be clearly labeled.

Overall Task Content Area:

Physical Science

Specific Knowledge Areas:

Structure of atoms/Radioactivity

Performance Expectations:

  • conducting investigations
  • gathering, organizing, and representing data
  • formulating conclusions from investigational data
  • applying scientific principles to develop explanations and solve new problems

National Science Education Standards:

12 B PS 1: Structure of atoms: Grades 9-12
1.4 Radioactive isotopes are unstable and undergo spontaneous nuclear reactions, emitting particles and/or wavelike radiation. The decay of any one nucleus cannot be predicted, but a large group of identical nuclei decay at a predictable rate. This predictability can be used to estimate the age of materials that contain radioactive isotopes.

12 A SI 1: Ability to do scientific inquiry: Grades 9-12
1.4 Formulate and revise scientific explanations and models using logic and evidence. Student inquiries should culminate in formulating an explanation or model. Models should be physical, conceptual, and mathematical. In the process of answering the questions, the students should engage in discussions and arguments that result in the revision of their explanations. These discussions should be based on scientific knowledge, the use of logic, and evidence from their investigation.

12 F SPSP 5: Natural and human-induced hazards: Grades 9-12
5.2 Human activities can enhance potential for hazards. Acquisition of resources, urban growth, and waste disposal can accelerate rates of natural change.

(Use the "hot" link on the PALS home page to check the full text of related National Science Education Standards, if desired.)

National Council of Teachers of Mathematics:

AL1: Understand patterns, relations and functions:
Grades 9-12 k. analyze functions of one variable by investigating rates of change, intercepts, zeros, asymptotes, and local and global behavior

AL3: Use mathematical models to represent and understand quantitative relationships:
Grades 9-12 f. draw reasonable conclusions about a situation being modeled

AL4: Analyze change in various contexts:
Grades 9-12 f. approximate and interpret rates of change from graphical and numerical data

DAP2: Select and use appropriate statistical methods to analyze data:
Grades 9-12 g. for univariate measurement data, be able to display the distribution, describe its shape, and select and calculate summary statistics

DAP3: Develop and evaluate inferences and predictions that are based on data:
Grades 9-12 f. use simulations to explore the variability of sample statistics form a known population and to construct sampling distributions

CNX3: Recognize and apply mathematics in contexts outside of mathematics:
Grades 9-12

REP3: Use representations to model and interpret physical, social, and mathematical phenomena:
Grades 9-12


At this station students should have:

3 cups
100 green beads
100 white beads
clock with second hand or digital watch
2 colored pencils (different colors) for graphing
pen or pencil

Advance Preparation:

The teacher will need to do no special prep work prior to the students performing this event.


  • Be careful.
  • Teachers and students should always exercise appropriate safety precautions and utilize appropriate laboratory safety procedures and equipment when working on science performance tasks.


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