With the Measuring Laboratory, students gain a better understanding of radioactivity and radiation. Students are able to visualize what is meant by radiation and background radiation.
Disciplinary Core Ideas (DCI, NGSS)
5-PS1-1, 3-5ETS1-2, MS-PS1-1, MS-PS1-4, MS-PS3-2, MS-ETS1-1, MS-ETS1-3, HS-PS1-8, HS-PS3-2, HS-PS4-1, HS-PS4-4, HS-ESS1-2, HS-ESS2-3, HS-ESS3-6
Time for Teacher Preparation
30-60 minutes – Clear the room of any unnatural radioactive sources. Create identifiable “locations” within the room – to correspond to the number of lab groups you will have. Code each of these locations in some way for easy reference.
30-60 minutes (1 Class Period)
Use as many Geiger counters as you have available. We will assume for this experiment that you are using Geiger counters which are not calibrated (they may not provide the same readings under the same circumstances). So, you may want to label each Geiger counter with a code number or letter; then, each group can record the code of the Geiger counter being used and use it for future activities.
- Geiger counters NOTE: digital read-out Geiger Counters give easier readouts for classroom use and more accurate measurements
- An assortment of objects with varying radioactivity, including some in each of three categories:
- Not detectably radioactive
- Just barely radioactive (“Vaseline glass”, thoriated welding rods, “depression green” glass, some fossils)
- Unambiguously radioactive (orange/red Fiestaware, certain lantern mantles, some uranium ore and minerals)
- Student Data Collection Sheets
Number each sample and record which category they fall into in a spreadsheet.
The students love testing the “hotter” items, but having the three categories of objects assures that everyone tests at least two each of clearly radioactive, marginally radioactive (would really need more counting time than available during lab to be sure), and essentially non-radioactive. The point is to have the students struggle with and face the uncertainty concerning whether or not items are radioactive.
- Students should use care when dealing with radioactive materials
- Students should wash their hands after this experiment
Science and Engineering Practices (NGSS)
- Ask questions and define problems
- Plan and Carry out investigation
- Analyze and interpret Data
- Use mathematics and computational thinking
- Construct Explanations
- Argue from Evidence
- Obtain, evaluate and communicate information
Cross Cutting Concepts (NGSS)
- Cause and Effect
- Scale, Proportion, and Quantity
- Energy and Matter: Flows, Cycles, and Conservation
- Structure and Function
- Stability and Change of Systems
- Familiarize students with the concept of background radiation.
- Determine the amount of background radiation present at a specific location.
- To define the terms radiation
- To become familiar with the different types of radiation
- To become familiar with operating a Geiger-Mueller counter
The key ideas for students to understand upon completing this lab are:
- There is background radiation wherever they are.
- Levels of background radiation vary somewhat from one location to another and from one moment to the next.
- Background radiation must be taken into account when measuring the radiation from an object.
- Uncalibrated Geiger counters may give slightly different counts in identical situations; however, they are useful for:
- determining that radiation is present.
- comparing radiation levels for locations or objects.
We live in a radioactive world, as did our earliest ancestors. The radiation in our world comes from many sources – cosmic radiation (outer space), terrestrial sources (the earth), radon in the air, etc. In addition, we live and work in buildings made from materials (stone, adobe, brick, concrete) which contain elements that are naturally radioactive. The amount of naturally occurring background radiation we experience varies, depending upon location.
Geiger counters will register the presence of some radiation even if you have not placed them near a known radiation source. This is a measure of the background radiation that is always present at a given location. In order to make meaningful measurements of the radioactive nature of specific objects or materials, we will need to know how much radiation is naturally present in the environment.
The difference between background radiation and the radiation measured near a specific object will give us the level of radiation due to the object. Although background radiation is quite steady on average, you would never conclude that by listening to or watching a Geiger counter. The amount of radiation will appear to vary, depending upon the specific time at which you take a measurement.
The covert theme of this lab is dealing with ambiguity. Because there is background radiation always giving a background signal, and a non-constant signal at that, measuring a sample for a minute or two (with ordinary Geiger counters) just cannot determine with certainty if the sample is weakly radioactive or not.
Teacher Lesson Plan
Before beginning, make sure students have some familiarity with the Geiger counter and how it will be used. Predetermine whether measurements are to be made with the “window” on the Geiger tube open or closed. Give students an overview of how and where to set the sensitivity level, etc.
- Have the students measure background counts for one minute.
- This is done by counting the number of “clicks” from the Geiger counter. It is not practical to make this measurement by reading the counts/min scale on the Geiger counter.
- Have each lab group enter the results of all the groups into the proper space on the table you provide.
- Ask students to examine the results. Do the results vary? If so, what is the lowest value and the highest value? What is the “range” of results? What are some possible reasons why the results might be different? Results will vary. Possible reasons include: inaccurate counting, inaccurate timing, slight variations in background radiation from location to location within the room, and/or differences between Geiger counters. There may be other suggestions from students — which you must evaluate.
- Ask students how they could try to eliminate some sources of error. They may suggest repeating the measurements to rule out inaccurate timing and counting. They may suggest removing any jewelry, etc.
- Have students run a second and third trial and enter only the data for their own group into the table.
- Ask the class: Do the results for your lab group vary from one trial to another? If so, why? What is the range for your own measurements? At this stage, students may have discovered that the results for their own group vary slightly in each trial.
Discuss this variation. Consider the possibility that errors were made during every measurement and discuss whether this is likely. Also, discuss the idea that the amount of background radiation present may actually be slightly different from one moment to the next — even though it has an “average” value. Refer to the water sprinkler analogy mentioned in the introduction. Have each group enter the “range” for their own measurements in the bottom row of the table.
- Regarding the counts they took, ask “Were the clicks always evenly spaced? OR, did the clicks sometimes cluster together with pauses between them?” Clicks are usually NOT evenly spaced. There are usually some “clusters” of clicks and some pauses. Discuss the possibility that this variation or “clustering” of clicks may have some impact on how long a time
period we use for measuring radioactivity levels. For example, using a really short time period might make measurements more prone to error than a longer time, especially if you did the “short period” measurement during a “pause” or during a “cluster” of clicks. 1. To illustrate, draw a clock face and let it represent a 60 second measurement. Then, make marks around the perimeter to represent when clicks are heard. This will give you clusters of marks and some empty spaces.
If someone takes a measurement in a specific period of 5 seconds, it can easily affect the count they get.
- Then, have the students enter the data for all of the groups into the table.
- Ask the students: Are there variations from group to group? If so, what are some possible reasons?
- Discuss possibilities: variations in Geiger counters, variations due to “location” in the room, etc.
- How could we determine if these differences are due to our Geiger counters being different or to differences within the room? You should realize when you begin this activity that these “uncalibrated” instruments are likely to give slightly different results under identical conditions and at the same time. However, it IS possible for there to be slight variations within the room. Proximity to a particular building material or exposure to some other radiation source, for example, may produce higher “background” readings in a specific location.
- There are several experimental approaches you and your students could use in resolving this issue.You could have each group make measurements at the same location and compare them. OR, each group could move to each of the identified locations and make readings for comparison purposes. Students may come up with other suggested solutions. Depending upon the time you want to allow and the sophistication level of your students, you can structure another set of measurements to provide an answer to the question above.
NOTE: If you are doing this activity in a one-period time slot, it is difficult to include measurement of background. Thus, most teachers use an average value for background, measured on a previous day. (Background varies little over time.)
Geiger Counter Resources:
How to attach speakers or head phones to a CD V-700
CD V-700 Instruction and Maintenance Manual
Troubleshooting for your CD-V700