Materials Included In Kit

Additional Materials Required

Safety Precautions

The radiation levels produced by the radioactive sources are extremely low (less than 1.0 μCi) and the sources are contained within sealed disks. Because the ionizing radiation “dose” is very low—similar to that obtained from watching television—no special safety precautions need to be taken. Observe normal laboratory safety guidelines. Please review current Safety Data Sheets before beginning this activity. Have students wash their hands thoroughly before leaving the laboratory.

Disposal

All materials may be saved for use in future laboratory activities.

Lab Hints

• The laboratory work for this experiment will require a double lab period (two 50-minute blocks of time). All materials are reusable. The Prelab Questions may be assigned as homework in preparation for lab. To facilitate the lab write-up, review the Prelab Questions during the lab period.
• Cut the paper sheets into 4 x 4 cm squares as needed to fit between the detector window and the source. The metal squares will stand up using the shielding supports. Tape the paper sheet to the detector to hold it in place.
• If time is running short, skip steps 11 and 12 for the alpha source.
• Other materials (e.g., wood, plastics or ceramics) can also be used to test their ability to absorb different types of radiation.
• Various software and interface systems may be used to collect the radiation data. Be sure to test the procedure before conducting the laboratory session. Modify the procedure as needed to reflect the operation of the detector and the software.
• This experiment can also be expanded to look at the combined effect of increased distance and shielding on the absorption of radiation.
• Various types of rulers may be used. The clear 12" rulers supplied with the kit have center “troughs” that provide convenient mounting sites for the sources when taking readings.
• Radioactive source kits containing sealed sources of alpha, beta and gamma radiation are available from Flinn Scientific (Catalog No. AP8796). Lantern mantles are also available from Flinn Scientific (Catalog No. AP5881). These mantles emit alpha and beta radiation, but no gamma radiation.
• A variety of “antique” sources of ionizing radiation may still be available in some location, although most are now very difficult to obtain. The original red Fiesta^® Ware dishes made by the Homer Laughlin Company in the 1930s and 40s utilized radioactive salts in the glaze. Authentic green “depression glass” from the 1930s also contain uranium. (Check that the glassware will fluoresce with a black light before assuming it is true Depression-era glass.) Luminous radium watch dials acquired a notorious reputation because the women who painted them were instructed to lick the paintbrushes!

Teacher Tips

• A tragic example of the dangers associated with nuclear radiation is the story of the so-called Radium Girls. In the 1920s, several U.S. companies produced watches and clocks that glowed in the dark. These timepieces had their hands and numbers painted with a mixture of glue, water and radium powder. The young women hired to paint these small parts were instructed to use their lips to make a fine point on the end of the paint brush. After a few years, a number of the women developed debilitating bone decay and anemia, with several dying. When five of these women sued the companies, the public outcry at their plight led to an investigation that showed radium poisoning as the cause of their grave conditions. Sadly, all five of the women died shortly after settling their lawsuit.

Answers to Prelab Questions

1. Why do different forms of nuclear radiation travel at different speeds? How is the speed of the radiation related to (a) its ability to “penetrate” matter and (b) its ability to ionize atoms as it travels through matter? Explain.

   All forms of nuclear radiation have approximately the same kinetic energy (1–5 MeV). The speed of different forms of radiation therefore depends on the mass of the particles or radiation. (a) Radiation that travels at the fastest speed (gamma radiation—the speed of light) will “penetrate” matter more deeply before it “hits” something and its energy is dissipated. (b) Radiation that travels more slowly (alpha radiation) is more likely to strike an atom and lose its energy very quickly.

2. Look up the density of aluminum versus lead in a reference manual. Which metal should be more effective in shielding or blocking nuclear radiation? Explain.

   The density of aluminum is 2.7 g/cm³, while the density of lead is 11.3 g/cm³. Lead will be a more effective shield than aluminum against nuclear radiation because there is more matter in the path of the radiation. Note: The critical variable is actually the electron density of the shielding material—the greater the electron density, the better the shielding will be.

3. X-rays are a form of ionizing radiation. They are high-energy photons (electromagnetic radiation) released when inner shell electrons that have been excited to higher energy levels release their excess energy. What form of nuclear radiation are X-rays similar to? What type of shielding is normally used to protect against X-rays?

   X-rays are similar to gamma radiation. Lead “aprons” are often used to protect against the harmful effects of X-rays.

Sample Data

Data Table A

Data Table B

Answers to Questions

1. Compare the background activity (number of counts per minute of background radiation) versus that of the alpha, beta and gamma sources in Part A. Is it necessary to “correct” the activity of the α, β and γ sources to take into account the level of background radiation? Explain.

   The background activity is about 100 times lower than the activity of the alpha, beta or gamma sources. It is not necessary to correct the activity of the sources for the low levels of background radiation.

2. What type(s) of shielding material can be used to absorb (a) alpha, (b) beta and (c) gamma radiation?
   1. Paper will completely absorb alpha radiation..
   2. Lead will completely absorb beta radiation..
   3. Lead will reduce the level of gamma radiation, but will not completely absorb it.
3. Which metal, aluminum or lead, is more effective in shielding against beta radiation? What is the reason for the difference in shielding ability of aluminum versus lead?

   Lead is a more effective shielding material than aluminum because it is more dense—there are more electrons in the path of the radiation.

4. Is it possible to completely stop gamma radiation using a sheet of metal? Would increasing the thickness of the metal stop more gamma radiation? Why or why not?

   A sheet of metal will not “stop” gamma radiation. Increasing the thickness of the metal should stop more gamma radiation, because there will be more atoms and electrons in the path of the radiation to absorb its energy. Even a lead block, however, will not completely stop gamma radiation.

5. Use arrows in the following diagram to show the ability of alpha, beta and gamma radiation to “penetrate” different types of shielding materials.
   {12646_Answers_Figure_2}
6. Prepare a graph of activity (counts per minute) on the y-axis versus the distance of the beta or gamma source from the detector on the x-axis.
   {12646_Answers_Figure_3}
7. Describe in words how the level of radiation from a radioactive source changes as the distance of the source from the detector increases.

   The activity of the radioactive source drops off sharply as the distance from the detector increases. At large distances (>20 cm), the radiation intensity or activity levels off and becomes indistinguishable from the background radiation.

8. Calculate the activity ratios at each of the following distances. Does the activity change by a constant amount when the distance from the source is doubled?
   {12646_Answers_Equation_1}
   The radiation intensity decreases by a relatively constant amount—about threefold—when the distance from the source to the detector is doubled. Note: Theoretically, the radiation intensity should decrease by a factor of four when the distance increases by a factor of two. (Radiation intensity follows an “inverse square” law.)
9. Based on the results obtained in Question 8, predict how the amount of radiation detected should change when the distance between the source and the detector is increased by a factor of four (e.g., from 5 cm to 20 cm). What was the actual activity ratio at 5 cm versus 20 cm?
   {12646_Answers_Equation_7}
   Radiation intensity appears to drop off exponentially (see the graph) as the distance increases. The activity should therefore decrease by a factor of 9–10 based on the ratios reported in Question 8. The mathematical reasoning is as follows: If 2ⁿ = 3, then n = 1.6, and 4ⁿ = 9. The actual activity decreased by a factor of 9.8. Note: According to the inverse square law, the activity should decrease by a factor of 4² = 16.
10. Explain how distance and shielding can be used together to protect workers from the harmful effects of gamma radiation.

    Neither distance nor shielding alone will completely absorb gamma radiation. However, combining the two methods of protection will absorb most of the gamma radiation and reduce the exposure of workers to safe levels.

11. (Optional) How could shielding be used to decide what type of radiation is emitted by an “unknown” radioactive source?

    First, test the ability of paper to stop or absorb the radiation. If paper absorbs the radiation, it must be an alpha source. Then test the radiation with a sheet of lead between the source and the detector to determine if the radiation is beta or gamma.

References

This experiment has been adapted from Flinn ChemTopic™ Labs, Volume 18, Nuclear Chemistry; Cesa, I., Ed., Flinn Scientific: Batavia, IL, 2005.

Introduction

Nuclear radiation is potentially very harmful to living organisms. In spite of the potential danger, however, nuclear radiation has been harnessed for many beneficial purposes, such as nuclear medicine and nuclear energy. How do workers in hospitals and nuclear power plants protect themselves from the harmful effects of nuclear radiation?

Concepts

• Alpha radiation
• Beta radiation
• Gamma radiation
• Penetrating power
• Ionizing radiation
• Shielding

Background

Alpha (α), beta (β) and gamma (γ) radiation are all forms of nuclear ionizing radiation. The characteristics of the different forms of nuclear radiation are summarized in Table 1. Alpha, beta and gamma radiation differ in their charge, mass, composition, penetrating power and ionizing ability.

Ionizing radiation deposits energy into body tissue, which can lead to cell damage. All people are constantly exposed to natural sources of “background” radiation, such as cosmic rays and radioactive elements in the Earth’s crust. People who work in nuclear medicine facilities and in nuclear power plants are exposed to greater amounts of radioactive materials. The health risk to these workers is minimized by (1) reducing the time they are exposed to radioactive sources, (2) increasing their distance from the sources and (3) absorbing the radiation with the proper type of shielding material.

Shielding refers to the ability of a material to absorb ionizing radiation. Different types of nuclear radiation require different types of shielding. When ionizing radiation strikes an atom, it transfers enough energy to the atom to strip it of an electron and create an ion. With each “strike,” the ionizing radiation loses energy. All forms of nuclear radiation have similar energies. The “speed” of the radiation, therefore, depends on its relative mass. Gamma radiation is pure electromagnetic radiation (no mass), traveling at the speed of light. Gamma radiation may thus travel great distances without striking an atom—it is the most penetrating form of nuclear radiation. Alpha particles are the most massive and therefore the slowest and least penetrating form of nuclear radiation. An alpha particle has the highest probability of hitting an atom as it travels through matter. Also, with each strike, an alpha particle will lose more of its kinetic energy than a beta particle or gamma ray. The amount of “shielding” required to absorb nuclear radiation is proportional to the penetrating power—gamma radiation requires the most shielding, alpha radiation the least. The ability of a material to absorb nuclear radiation depends on the density and the thickness of the material. (The more electrons and nuclei there are in the path of the incoming radiation, the more effective the material will be in “stopping” the radiation.)

Experiment Overview

The purpose of this experiment is to compare the properties of alpha, beta and gamma radiation. The activity (counts per minute) of low-level α, β and γ sources will be measured using a Geiger counter, which “counts” the number of atoms ionized by nuclear radiation. The relative penetrating power of α, β and γ radiation will be investigated by measuring how the recorded activity (counts per minute) changes as different materials are placed between the source and the detector. The effectiveness of different shielding materials will also be determined.

Materials

Prelab Questions

1. Why do different forms of nuclear radiation travel at different speeds? How is the speed of the radiation related to (a) its ability to “penetrate” matter and (b) its ability to ionize atoms as it travels through matter? Explain.
2. Look up the density of aluminum versus lead. Which metal should be more effective in shielding or blocking nuclear radiation? Explain.
3. X-rays are a form of ionizing radiation. They are high-energy photons (electromagnetic radiation) released when inner shell electrons that have been excited to higher energy levels release their excess energy. What form of nuclear radiation are X-rays similar to? What type of shielding is normally used to protect against X-rays?

Safety Precautions

The radiation levels produced by the radioactive sources are extremely low (less than 1.0 μCi) and the sources are contained within sealed disks. Because the ionizing radiation “dose” is very low—similar to that obtained from watching television—no special safety precautions need to be taken. Observe normal laboratory safety guidelines. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands after handling any radioactive materials.

Procedure

Part A. Radiation Shielding

1. Set up the Geiger counter as shown in Figure 1. Set the detector horizontally on the bench and place a metric ruler in front of the detector window.
   {12646_Procedure_Figure_1}
2. Connect the Geiger counter to the interface system, and connect the interface system to the computer or calculator.
3. Turn on the computer or calculator and open the data collection program for the radiation detector.
4. Set a data collection interval of 60 seconds—this will be the length of time the activity of the radioactive source will be “counted.” Set a sampling rate or “count interval” of 10 seconds/sample (0.1 samples/second).
5. (Optional) Adjust the highest value of radiation (counts) on the y-axis as needed.
6. Determine the “background” activity when there is no radioactive source placed in front of the radiation detector:
   1. Select Collect from the data collection program to begin collecting data.
   2. Wait 60 seconds to complete the data collection interval. Record the number of background counts per minute in Data Table A. Save the experiment file as desired.
   3. Return to the beginning of the data collection program and repeat steps (a) and (b) two more times, for a total of three trials.
7. Measure the activity of the alpha source: Place the alpha source approximately 1 cm from the radiation detector, with the unlabeled side of the disc facing the detector. Press Collect to begin collecting data, and wait 60 seconds for the program to complete the data collection interval. Record the number of counts per minute in Data Table A.
8. Repeat step 7 two more times for a total of three trials.
9. Place a single sheet of paper between the alpha source and the detector and measure the activity (counts per minute) as before. Try to keep the source in the same position with respect to the radiation detector. Record the number of counts per minute in Data Table A.
10. Repeat step 9 two more times for a total of three trials.
11. Remove the paper and place a sheet of aluminum between the alpha source and the detector. Measure the activity (counts per minute) three times and record the data in Data Table A.
12. Remove the aluminum and place a piece of lead between the alpha source and the detector. Measure the activity (counts per minute) three times and record the data in Data Table A. Save the experiment file as desired.
13. Repeat steps 7–12 twice more using first the beta source and then the gamma source.

Part B. Effect of Distance on Radiation Intensity

14. Reset the data collection interval for 60 seconds, if necessary.
15. Place the beta or the gamma source 2 cm from the radiation detector.
16. Press Collect to begin collecting data and wait 60 seconds for the program to complete the data collection.
17. Record the activity (counts per minute) in Data Table B. Save the experiment file as desired.
18. Repeat steps 16 and 17 two more times for a total of three measurements of the radioactivity at 2 cm.
19. Move the beta or gamma source 5 cm away from the radiation detector. Repeat steps 16 and 17 three times to obtain three measurements of the activity at 5 cm. Record all data in Data Table B.
20. Repeat step 19 at distances of 10 cm and 20 cm. Record all data in Data Table B.

Student Worksheet PDF

12646_Student1.pdf