In this lab, students will investigate properties of three types of ionizing radiation—alpha, beta and gamma radiation. Students will have the opportunity to design their own procedures to explore the relationship between distance and radiation intensity.
AP Chemistry Curriculum Framework
This lab supports the following learning objectives:
- Big Idea 1: The chemical elements are fundamental building materials of all matter, and all matter can be understood in terms of arrangements of atoms. These atoms retain their identity in chemical reactions.
- 1.12 The student is able to explain why a given set of data suggests, or does not suggest, the need to refine the atomic model from a classical shell model with the quantum mechanical model.
- 1.13 Given information about a particular model of the atom, the student is able to determine if the model is consistent with specified evidence.
By the end of this lab, students should be able to
- Detect background radiation using a radiation detector and record its intensity.
- Recognize that some objects omit ionizing radiation
- Distinguish between the forms of ionizing radiation, and identify the properties of each.
- Design a procedure for investigating the relationship between distance and radiation intensity.
- Graph collected data to analyze the relationship between distance and radiation intensity.
- Recognize the penetrating ability of different radiation sources.
This lab supports students’ understanding of
- Nuclear Chemistry
- Ionizing Radiation
- Alpha, Beta, Gamma particles
Teacher Preparation: 30 minutes
Lesson: 60 minutes
(Per student group)
- Radiation Counter, such as a Geiger-Müller counter
- Alpha radiation source
- Beta radiation source
- Gamma radiation source
- Forceps or crucible tongs (for handling radiation sources)
- Meter stick
- Cardboard or index card
- Glass sheet (5 cm × 5 cm × 0.3 cm)
- Lead sheet (5 cm × 5 cm × 0.1 cm)
- Disposable gloves
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- Handle all radioactive samples with great care, including wearing protective gloves.
- With proper handling, radioactive materials in this investigation pose no danger to you. Nuclear materials are strictly regulated by state and federal laws. The radioactive sources you will use emit only very small quantities of radiation; using them requires no special license.
- Do not allow the radiation counter to come in direct contact with the radioactive material.
- Check your hands with a radiation monitor before you leave the laboratory.
- This resource could be used as a post-AP Chemistry exam activity.
- Only a few detectors are sensitive to all three types of ionizing radiation. Alpha radiation is commonly the hardest to detect. Thus, depending on available equipment, you may have difficulty demonstrating alpha-particle detection. If equipment is in short supply, you may demonstrate this procedure. This activity addresses several important concepts, so students should at least observe a demonstration.
- The American Nuclear Society conducts workshops, some of which distribute Geiger- Müller counters to schools. Your school’s physics department or local college or university physics department may also have Geiger counters available to lend.
- Adjust laboratory procedures to the apparatus. Not all detectors will measure all forms of radiation.
- Radioactive sources sealed in plastic discs are available from most commercial school-science suppliers.
- Be sure that no extraneous radioactive materials come in contact with your radiation counter. The counter can become contaminated with radiation and record higher-than-normal background readings.
- Check the background radiation level before students begin the investigation.
- Cover lab tables with butcher paper for these nuclear investigations.
- The use of gloves in this investigation is not only to shield students from radiation but also to protect the sample from contamination. With sealed commercial sources, there is little risk of this type of contamination.
- If possible, it is better to count clicks rather than just recording an average meter reading.
- Since the radioactive sources used will probably be weak, the total time needed may have to be extended from 30 seconds to a few minutes.
- If students’ readings vary greatly, instruct them to take additional readings.
- Avoid overexposure to the radioactive materials, to present as little risk to yourself and students as possible.
Answers to the Student Investigation
1. Student graphs should show an inverse square relationship (as discussed in Interpreting Evidence Question 2).
1. alpha < beta < gamma. Students should cite data supporting their conclusions.
2. a. Intensity decreased by a factor of four.
b. Yes. At four times the original distance, intensity is approximately one-sixteenth of the original intensity, or one fourth of the doubled-distance intensity.
c. The intensity varies inversely with the square of the distance
(I = k/d2).
3. a. Gamma radiation is most likely to penetrate. Alpha is blocked by paper, and beta by thick plastic, which would imply that body tissue would stop alpha and beta.
b. The alpha particles would have to be in direct contact with living tissue to damage the organ.
4. a. Lead is most effective. It reduced the levels of all three types of radiation.
b. Cardboard is least effective. It can only reduce alpha (the least penetrating) radiation.
5. Density and thickness
6. a. Students should note that their data are consistent with the figure.
b. The intensity of radiation decreases by the inverse square law. As the distance doubles, the intensity decreases by 1⁄4.
c. The area of the green box would be 64 cm2.
7. a. Questions vary but should be testable and match the variables students investigated.
b. Students should identify the variable they changed (independent), the variable they measured (dependent), and how they controlled the remaining variables.
c. The claims or conclusions should match student results and be supported by cited observations or data.
Reflecting on the Investigation
8. a. Lead
b. Lead is very dense and can stop high-energy radiation such as gamma rays and X-rays.
9. Answers will vary widely. An example might be: “How does the distance between the source and the shield change the amount of radiation that penetrates the shield?”
For the Student
Preparing to Investigate
You should already have read about ionizing and non-ionizing radiation and should be familiar with the term background radiation. This investigation will allow you to determine in more detail some properties of three types of ionizing radiation—alpha, beta, and gamma radiation.
To frame this investigation, review what you have already learned:
- Background radiation is always present (and can be detected).
- Some objects emit ionizing radiation, which can be measured with the same devices used to measure background radiation.
- Alpha particles, beta particles, and gamma rays are all forms of ionizing radiation, but they have properties that differ from each other. For example, alpha particles are positively charged, beta particles are negatively charged, and gamma rays have no electrical charge.
Also consider these two claims: “alpha radiation will cause more harm internally to living organisms than will the same quantity of gamma radiation” and “gamma rays . . . penetrate deeply into human tissue.” Given these claims, you may want to know more about the likelihood of alpha particles penetrating human tissue—how does their penetrating ability compare to that of gamma rays? You may also be wondering how beta particles compare in terms of their penetrating ability.
For the type of radiation with best penetrating ability, you might wonder how distance from the radiation source affects the dose you receive. Overall, you may be most concerned with how to shield yourself from radiation or may wonder why you are required to wear a lead apron when receiving dental x-rays.
This investigation will allow you to address these questions. It is important to note that with proper handling, radioactive materials in this investigation pose no danger to you. Nuclear materials are strictly regulated by state and federal laws. The radioactive sources you will use emit only very small quantities of radiation; using them requires no special license. Nevertheless, you should handle all radioactive samples with great care, including wearing protective gloves. Do not allow the radiation counter to come in direct contact with the radioactive material. Check your hands with a radiation monitor before you leave the laboratory.
Read the procedure described in Gathering Evidence, Part I. What scientific question should you be able to answer after you have gathered all of your evidence? Construct a data table suitable for recording all relevant data.
- Before you begin, put on your goggles, and wear them properly throughout the investigation.
- Set up the apparatus shown in the figure above. There should be space between the source and the detector for several sheets of glass or metal.
- Turn on the counter; allow it to warm up for at least 3 min. Determine the intensity of background radiation by counting the clicks for one minute without any radioactive sources present. Record this background radiation value in counts per minute (cpm) in your data table.
- Put on protective gloves. Using forceps, place a gamma-ray source on the ruler at a point where it produces a nearly full-scale reading. Record the distance between the source and the detector.
- Observe the meter for 30 s and estimate the number of counts per minute detected over this period. Record this approximate value. Then subtract the background reading from that value and record the corrected results.
- Without moving the radiation source, place a piece of cardboard (or an index card) between the detector and the source, as shown in Figure 2.
- Observe the meter for 30 s. Record the typical reading. Then correct the reading for background radiation and record the corrected result in your data table.
- Repeat Steps 6 and 7, replacing the cardboard with a glass or plastic sheet.
- Repeat Steps 6 and 7, replacing cardboard with a lead sheet.
- Repeat Steps 4 through 9, using a beta-particle source.
- Repeat Steps 4 through 9, using an alpha-particle source.
Part II: Effect of Distance on Intensity
In Part I, you measured radiation intensity from a single distance from the source. What do you think is the relationship between distance and radiation intensity? You will design your own procedure for investigating this question.
- Write down your prediction about the relationship between distance and intensity. For instance, do you expect doubling the distance lead to half the intensity reading?
- Design a procedure that will allow you to test your prediction. Think about how many distances you should test and what data you will need to record. Write down your detailed procedure and show it to your teacher before proceeding.
- Carry out your investigation. Be sure to record all relevant data.
Part III: Further Investigations
In this part you will conduct an additional investigation of your own design.
- Note the materials your teacher has provided to conduct this investigation.
- Think about one or two additional questions that relate to, or extend, the investigations you have already conducted using these ionizing radiation sources.
- Choose one question that you can address using the available equipment and materials. Write down the scientific question that you will investigate.
- Propose and write down a detailed procedure for your investigation.
- Conduct your investigation. Be sure to record all relevant data.
- Graph your data from Part II, plotting corrected cpm values on the y-axis and distances from the source to the detector (in cm) on the x-axis.
- Order the three types of ionizing radiation from “least penetrating ability” to “greatest penetrating ability.” Support your answer with data or observations from Part I.
- Interpret your data from Part II.
a. By what factor did the intensity of radiation (measured in counts per minute) change when the initial distance was doubled?
b. Did this same factor apply when the distance was doubled again?
c. State a mathematical relationship between distance and intensity.
- It has been claimed that “alpha radiation will cause more harm internally to living organisms then will the same quantity of gamma radiation.”
a. Based on your investigations, which type of radiation is more likely to penetrate internal organs if the source of radiation is outside the body? Support your answer with data or observation.
b. Describe what would need to happen for alpha particles to cause damage to internal organs.
- Of the shielding materials tested, which do you conclude is the
a. most effective in blocking radiation? Cite supporting evidence.
b. least effective in blocking radiation? Cite supporting evidence.
- Based on your observations, what properties of a material appear to affect its ability to be penetrated by radiation?
- Figures like the one below are often used to illustrate the relationship between distance from a radiation source and its intensity.
a. Does the information in this figure fit with the data you collected in Part II?
b. Write a caption for the figure that would help someone else interpret this diagram.
c. What would the diagram look like at a total distance of 15 cm from the source? Explain your reasoning.
- If you conducted an additional investigation in Part III:
a. State the scientific question you were addressing in your investigation.
b. Summarize how you designed your experiment to address your question.
c. State your conclusions and/or claims. Support your conclusions using data or observations from your investigation.
Reflecting on the Investigation
- A patient receiving an X-ray is covered by a protective shield.
a. What material would be a good choice for this apron?
- Whether or not you conducted an additional investigation, propose one additional question you have about alpha, beta, and gamma radiation.