Lesson

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.

Gathering Evidence

Part I:

1. Before you begin, put on your goggles, and wear them properly throughout the investigation.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. Repeat Steps 6 and 7, replacing the cardboard with a glass or plastic sheet.
9. Repeat Steps 6 and 7, replacing cardboard with a lead sheet.
10. Repeat Steps 4 through 9, using a beta-particle source.
11. 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.

1. 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?
2. 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.
3. 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.

1. Note the materials your teacher has provided to conduct this investigation.
2. Think about one or two additional questions that relate to, or extend, the investigations you have already conducted using these ionizing radiation sources.
3. Choose one question that you can address using the available equipment and materials. Write down the scientific question that you will investigate.
4. Propose and write down a detailed procedure for your investigation.
5. Conduct your investigation. Be sure to record all relevant data.

Analyzing Evidence

1. 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.

Interpreting Evidence

1. 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.
2. 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.
3. 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.

Making Claims

1. 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.
2. Based on your observations, what properties of a material appear to affect its ability to be penetrated by radiation?
3. 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.
4. 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

1. A patient receiving an X-ray is covered by a protective shield.
   a. What material would be a good choice for this apron?
   b. Why?
2. Whether or not you conducted an additional investigation, propose one additional question you have about alpha, beta, and gamma radiation.