Materials Included In Kit

Additional Materials Required

Safety Precautions

The radiation levels produced by the radioactive sources are extremely low (less than 0.1 μCi) and the sources are contained within sealed disks. Because the ionizing radiation “dose” is very low—lower than that of a dental X-ray—no special safety precautions need to be taken. Observe normal laboratory safety guidelines. Lead solid is a probable carcinogen when ingested or inhaled as fine particles. Do not allow students to cut the metal sheets.

Disposal

All materials may be saved for use in future laboratory activities. Sealed radiation sources used for educational purposes as well as consumer products, such as lantern mantles that contain thorium, provide very low levels of radiation exposure and do not pose a threat to public health. These devices are exempt from Nuclear Regulatory Commission (NRC) requirements for storage, handling and disposal. Exempt-quantity sources that become useless may be placed in the regular trash. Remove labels and other identifying marks prior to disposal to avoid any confusion if found by someone else.

Lab Hints

• The laboratory work for this experiment may be completed within a 2-hour lab period.
• Cut the metal absorber sheets into 4 x 4 cm squares as needed to fit between the detector window and the source. The metal sheets will stand up against the detector window and thus do not need to be clamped in place. Tape the paper sheet to the detector to hold it in place.
• Other materials, such as wood, plastics or ceramics, can also be used to test their ability to absorb different types of radiation.
• Various radiation detectors as well as software and interface systems may be used to collect data. Be sure to test the procedure before conducting the laboratory session. Modify the procedure as needed to reflect the operation of the radiation detector and any software or programs.
• This experiment can also be expanded to look at the combined effect of increased distance and shielding on the absorption of radiation.
• 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. The hands and numbers on these timepieces were 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.
• The Nuclear Regulatory Commission Definition of Exempt Quantities may be found at NRC 10 CFR 30. The definition includes sealed sources such as those used in this activity, and contains the disposal guidelines discussed in the Disposal section.

Further Extensions

Supplementary Information

Units of Radioactivity and Radiation
Nuclear radiation refers to the high-energy particles and electromagnetic radiation emitted during nuclear decay. This high energy radiation can cause very grave biological damage. What are the units for measuring nuclear radiation and its potential danger?

Activity is a measure of the radioactivity of a substance—it corresponds to the number of nuclei that disintegrate or decay per unit of time. There are two commonly used units of activity:

• Curie (Ci). The curie, named after Marie Curie, represents the activity of 1 gram of radium-226.

  1 Ci = 3.7 x 10¹⁰ disintegrations per second

• Becquerel (Bq). The historic curie unit is being replaced in the literature by the SI unit, the becquerel, named after the discoverer of radioactivity, Henri Becquerel.

  1 Bq = 1 disintegration per second

Because the curie is a large value of activity, low levels of nuclear radiation are typically measured in millicuries (mCi), microcuries (μCi) or picocuries (pCi).

The activity of a radioactive source does not tell us the actual energy of its emitted radiation or the biological damage the radiation would produce. Two units of absorbed radiation are:

• Rad (Radiation absorbed dose). 1 rad equals 0.01 joules of energy absorbed per kilogram of tissue.
• Gray (Gy). This is the SI unit of absorbed energy. One gray represents 1 joule of absorbed energy per kilogram of tissue. 1 Gy = 100 rads.

To convert the activity of an isotope to rads or grays, the activity must be multiplied by the energy of the specific radiation produced by that isotope. These energy values can be found in tables of nuclear data. (Values may be found in The Handbook of Chemistry and Physics.)

Rads are used to describe the amount or dose of energy that will be absorbed. However, equal doses of different forms of nuclear radiation produce different biological effects. To take this difference into account, two units are used to assess the dose effect, or biological damage, of nuclear ionizing radiation. These dose equivalent units are:

• Rem. 1 rem equals the amount of absorbed radiation producing the same biological effect as 1 rad of therapeutic X-rays.
• Sievert (Sv). This is the new SI dose equivalent unit. One Sv equals 100 rems.

The dose equivalence of different radiation sources is calculated by multiplying the absorbed radiation by the relative biological effectiveness (RBE) of the particular ionizing radiation. The following estimates of RBE, called quality factors, are used to calculate dose equivalence.

Radiation dose calculations take into account the activity of the source, mode of decay of the radionuclide, energy of the radiation and quality factor of the radiation.

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 speed of light will “penetrate” matter more deeply before it “hits” something.
   b. Radiation that travels more slowly is more likely to strike an atom and transfer its energy, resulting in ionization.

2. Look up the density of aluminum versus lead. 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 is a more effective shield than aluminum against nuclear radiation because there is more matter in the path of the radiation.

3. X-rays are high-energy photons (electromagnetic radiation) that are released when metals are excited at high voltages and then release their excess energy. What form of nuclear radiation are X-rays similar to? What type of shielding protects against X-rays?

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

4. We are surrounded by background radiation. Sources of natural radiation include cosmic rays, uranium in soil and rocks, radon in the air and carbon-14 as well as potassium-40 in all living things. Technology has created additional sources of background radiation. Nuclear medicine encompasses traditional X-rays, as well as radiation therapy and newer forms of diagnosis such as CAT scans. Televisions and computer monitors also emit low energy X-rays. The average radiation dose per person in the United States is 360 millirems per year. Fill in the missing values in the following table to estimate your exposure to background radiation.
   {14043_PreLabAnswers_Table_3}

Sample Data

Laboratory Report

Radiation Shielding: Number of Counts per Minute

Effects of Distance on Beta Radiation Intensity

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 with no shielding. 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 at least 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 atoms 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.

5. Use arrows in the following diagram to show the ability of alpha, beta and gamma radiation to “penetrate” different types of shielding materials.
   {14043_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.
   {14043_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?
   {14043_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?
   {14043_Answers_Equation_2}
   Radiation intensity appears to drop off exponentially (see the graph) as the distance increases. The activity should 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. How can 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.

12. The following calculations illustrate the radiation exposure from working with the sealed radioactive sources in this experiment. The gamma source is cobalt-60, having an activity of 1.0 μCi, and the alpha source is polonium-210, with an activity of 0.1 μCi. Calculate the maximum radiation dose in millirems (mrems) for an average 150 lb (68 kg) individual working with the Co-60 source for one hour.
    {14043_Answers_Table_6}
    {14043_Answers_Equation_3}

Introduction

Nuclear radiation is potentially very harmful to living organisms. Despite its potential danger, nuclear radiation has been harnessed for many beneficial purposes, including 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) monitoring and reducing the amount of 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 atoms 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-Müller radiation detector, 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 high-energy photons (electromagnetic radiation) that are released when metals are excited at high voltages and then release their excess energy. What form of nuclear radiation are X-rays similar to? What type of shielding protects against X-rays?
4. We are surrounded by background radiation. Sources of natural radiation include cosmic rays, uranium in soil and rocks, radon in the air and carbon-14 as well as potassium-40 in all living things. Technology has created additional sources of background radiation. Nuclear medicine encompasses traditional X-rays, as well as radiation therapy and newer forms of diagnosis such as CAT scans. Televisions and computer monitors also emit low energy X-rays. The average radiation dose per person in the United States is 360 millirems per year. Fill in the missing values in the following table to estimate yourexposure to background radiation.
   {14043_PreLab_Table_2}

Safety Precautions

The radiation levels produced by the radioactive sources are extremely low (less than 0.1 μCi) and the sources are contained within sealed disks. Because the ionizing radiation “dose” is very low—lower than that of a dental X-ray—no special safety precautions need to be taken. Observe normal laboratory safety guidelines. Lead solid is a probable carcinogen when ingested or inhaled as fine particles. Do not cut the metal sheets provided by the instructor.

Procedure

Radiation Shielding

1. Set up a Geiger-Müller radiation detector or digital radiation meter as shown in Figure 1. Set the detector horizontally on the bench and place a metric ruler in front of the detector window.
2. (Optional) Depending on the equipment being used, the radiation detector may be connected to an interface system and a computer, tablet or calculator.
3. (Optional) 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. Determine the “background” activity when there is no radioactive source placed in front of the radiation detector:
   1. Begin collecting data.
   2. Wait 60 seconds to complete the data collection interval. Record the number of background counts per minute. Save the experiment file as desired.
   3. Repeat steps (a) and (b) two more times, for a total of three trials.
6. Measure the activity of the alpha source: Place the alpha source 1 cm from the radiation detector, with the underside of the disc facing the detector. Begin collecting data, and wait 60 seconds for the program to complete the data collection interval. Record the number of counts per minute.
7. Repeat step 6 two more times for a total of three trials.
8. 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.
9. Repeat step 8 two more times for a total of three trials.
10. 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.
11. 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. Save the experiment file as desired.
12. Repeat steps 6–11 twice more using first the beta source and then the gamma source.

Effect of Distance on Radiation Intensity

13. Reset the data collection interval for 60 seconds, if necessary.
14. Place the beta or the gamma source 2 cm from the radiation detector.
15. Begin collecting data and wait 60 seconds for the program to complete data collection, if applicable.
16. Record the activity (counts per minute). Save the experiment file as desired.
17. Repeat steps 15 and 16 two more times for a total of three measurements of the activity at 2 cm.
18. Move the beta or gamma source 5 cm away from the radiation detector. Repeat steps 15–17 three times to obtain three measurements of the activity at 5 cm. Record all data.
19. Repeat step 18 twice more at distances of 10 cm and 20 cm. Record all data.
    {14043_Procedure_Figure_1}

Student Worksheet PDF

14043_Student1.pdf