Materials Included In Kit

Additional Materials Required

Safety Precautions

Lantern mantles produce extremely low levels of radiation (less than 0.1 μCi) and are considered safe for consumer use. Wear gloves when handling the lantern mantle and wash hands thoroughly with soap and water afterward. Do not light the mantle and avoid inhaling mantle dust. Isopropyl alcohol is slightly toxic by ingestion and inhalation and is a flammable solvent—do not use near flames or other sources of ignition. Dry ice is extremely cold and may cause frostbite. Never touch dry ice with bare skin. Wear insulated gloves when handling dry ice. Wear chemical splash goggles whenever working with chemicals, glassware or heat in the chemical laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The dry ice may be allowed to evaporate in a well-ventilated area. The blotting paper should be allowed to dry, and the entire cloud chamber kit may then be reused. Store the lantern mantles in a plastic bag and save for repeat use—they will last indefinitely. (The half-life for thorium-232 is 1.4 x 10¹⁰ years.)

Lab Hints

• The setup and observation period for this experiment is 20–30 minutes. If more cloud chambers are needed, the experiment may be scheduled concurrently with another experiment, a demonstration or an assignment worksheet. Additional cloud chambers and lantern mantles are available from Flinn Scientific (Catalog Nos. AP8807 and AP5881, respectively).
• If large ceramic squares are not available, use a towel, several layers of paper towels or sets of smaller ceramic squares.
• A variety of “antique” sources of ionizing radiation may still be available in some locations, 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 contained 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!
• Radioactive source kits containing sealed sources of alpha, beta and gamma radiation (see Flinn Scientific Catalog No. AP8796) may be used to help differentiate among cloud tracks produced by different types of radiation. Smoke detectors also contain a radioactive source but it is generally not recommended that students take apart smoke detectors to get at the americium source.
• Methyl, ethyl and isopropyl alcohol may all be used to produce vapor trails in the cloud chamber. We found that the best tracks were obtained using isopropyl “rubbing” alcohol, although other groups recommend using ethyl alcohol. We do not recommend using methyl alcohol because of its toxicity and flammability.
• Some trial and error is usually required to obtain optimum conditions for evaporation and condensation in the cloud chamber. If no tracks are present when there is a misty layer on the bottom of the chamber, try warming the top of the chamber with your hand. This will cause more alcohol to evaporate. Also, for best results, turn off the lights in the room, if possible, and shine the light from the side only, not through the top of the chamber.

Teacher Tips

• It may be helpful to review a list of various sources of natural radiation with students before beginning this activity. Many students may be afraid to work with nuclear radiation, fearing that all radiation is unnatural and harmful. The review may help students understand how they are exposed to natural radiation all the time in amounts far exceeding the radiation emanating from the lantern mantles used in this experiment. Having said that, remember that all of us are advised to avoid unnecessary exposure to nuclear radiation, including medical and dental X-rays, unless absolutely needed. A worksheet on sources of natural radiation is included in the Supplementary Information on the Teacher PDF.
• Challenge students to devise a method for distinguishing between alpha and beta radiation produced by the lantern mantle. Observe the tracks produced when the mantle is uncovered, then wrapped in a piece of paper. The paper should absorb the alpha particles, leaving only the beta particles to produce trails. Place a magnet to the side or underneath the chamber and observe if any of the particle tracks are affected. (Remember, however, that the tracks are due to ions produced by the alpha and beta particles.)
• Older cloud chambers often came with alpha sources attached to small probes. These sources are no longer available.
• The official website of the Nobel Foundation (www.nobelprize.org) is a wonderful electronic museum of information. The website includes fact-checked biographies, official award citations and presentation speeches for all of the award winners. Charles T. R. Wilson’s Nobel lecture may be downloaded from the website as a PDF file—it contains original photographs of the cloud tracks produced by different kinds of radiation.

Answers to Prelab Questions

1. What kinds of materials can be used to shield or protect an object from alpha and beta radiation? What safety precautions are used in this experiment to protect against radiation released by the lantern mantle?

   Alpha particles will not penetrate through paper or skin—an object may be shielded from alpha particles simply by placing a piece of paper between the source and the object. In the same way, our skin acts as a shield against alpha particles. Beta particles will not penetrate through wood, metal or layers of clothing. Wearing gloves and a lab coat if possible will help protect against beta-particle radiation. The only way for alpha particles to penetrate our bodies in this experiment is by ingestion or inhalation. The safety precautions also specify washing hands thoroughly after handling the lantern mantle to prevent particles remaining on our hands from entering our mouths.

2. Complete the following table summarizing the properties of alpha, beta and gamma radiation. (See the Background section.)
   {12612_PreLabAnswers_Table_2}
3. Compare the penetrating power and the ionizing ability of alpha and beta particles. Which type of radiation should produce longer tracks in the cloud chamber? Which type of radiation will probably give thicker tracks due to the production of lots of ions?

   Beta particles will travel farther through air than alpha particles, but alpha particles will produce more ions than beta particles. Beta particles should produce long, thin tracks. Alpha particles will probably give short, thick tracks.

Sample Data

Answers to Questions

1. Describe the different kinds of tracks observed in the cloud chamber. Using the properties of alpha particles and beta particles, predict which tracks were probably produced by each particle.

   There are two main types of tracks. Short, thick, straight trails (about 1 cm long) are probably produced by alpha particles, which do not travel very far through air. Alpha particles collide more often with air molecules and produce more ions in the process. Thin, straight tracks—about 2–3 cm long—are probably due to beta particles, which may travel farther through air than alpha particles but produce fewer collisions and thus fewer ions for liquid to condense on. Note: The tracks due to beta particles may curve at the end due to scattering when they bounce off other particles. Gamma rays are difficult to see in a cloud chamber. When there is no radiation source, cosmic rays may enter the chamber and produce thin misty trails. These are probably the very faint tracks which seem to suddenly appear and then disappear.

2. Explain why some tracks may have been observed only very close to the radioactive source, while other types of tracks were observed farther away from the source.

   Alpha tracks should be observed near the radiation source, because they do not travel very far through the air. They rapidly lose their kinetic energy due to collisions with molecules in the air. Beta particles are smaller and faster moving and will travel farther through air. These tracks are observed further from the radiation source.

3. What is the purpose of using dry ice to cool the cloud chamber? What would happen if the entire chamber, including the sides and the blotting paper, were too cold? What would happen if the cloud chamber were too warm?

   The bottom of the chamber must be cold to produce a layer of cold air that will become supersaturated with alcohol vapor. The vapor will then condense if there are ions in the air to act as “seeds” for droplets to form. If the entire chamber is too cold, however, the alcohol will not evaporate from the blotting paper lining the sides of the chamber. There will not be enough vapor in the air to condense. Finally, if the chamber is too warm, the alcohol vapor will not condense.

4. Explain why rubbing the top of the cloud chamber with silk may be used to refresh the cloud chamber, causing the tracks to disappear and then reappear.

   Rubbing the chamber with silk produces static electricity, which attracts the ions and neutralizes them. The cloud tracks disappear when the air in the chamber is briefly cleared of ions in this way.

5. Some cloud tracks will be observed in the chamber even in the absence of an external radioactive source. What is the possible origin of these tracks?

   Cosmic rays in the atmosphere may enter the chamber. They will produce very faint misty trails.

6. Because of its low penetrating power, alpha radiation is not an external hazard. However, inhaling radioactive “dust” that emits alpha particles is very dangerous. Explain.

   Alpha particles are not an “external” radiation hazard because they do not penetrate skin. Inhaling or ingesting alpha particles, however, is extremely dangerous due to their ionizing ability. Once inside the lungs, alpha particles do not have to travel far to cause damage to lung tissue.

Teacher Handouts

12612_Teacher1.pdf

References

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

Introduction

A cloud chamber is a simple device for detecting low levels of nuclear radiation. Just as “seeding” a cloud with crystals produces rain or snow, passing ionizing radiation through a chamber saturated with vapor leaves a trail of liquid droplets in its wake.

Concepts

• Nuclear radiation
• Alpha radiation
• Beta radiation
• Gamma radiation
• Ionizing radiation
• Cloud chamber

Background

Radioactivity is defined as the spontaneous decay or disintegration of the nucleus of an atom. Radioactive decay changes the number of protons and neutrons in the nucleus of an atom and results in the release of nuclear radiation—high-energy particles and electromagnetic radiation. Different pathways for radioactive decay give rise to different types of nuclear radiation, such as alpha particles, beta particles and gamma rays.

Very large, unstable nuclei usually decay by emitting alpha particles. An alpha (α) particle has a nuclear charge of +2 and a relative mass of 4 amu (atomic mass units), identical to a helium nucleus in composition (two protons and two neutrons). The symbol for an alpha particle is

where the 4 superscript represents the particle mass, in amus, and where the subscript 2 represents the particle’s nuclear charge. Alpha particles are heavy and relatively slow moving—they do not travel very far in air and will not “penetrate” paper or skin. Lighter nuclei often decay by emitting beta particles. A beta (β) particle has a charge of –1 and a relative mass equal to that of an electron. Although beta particles are equivalent to electrons, they arise via a nuclear process in which a neutron in the nucleus of an atom decays to form a proton and an “energetic” electron, which then escapes from the nucleus. The symbol for a beta particle is Fast-moving beta particles, being much smaller and lighter in mass than alpha particles, have a penetrating power about ten times greater than that of alpha particles. A block of wood, a sheet of metal or layers of clothing will shield an object from beta-particle radiation. Gamma radiation is pure electromagnetic radiation (high-energy photons). The symbol for a gamma ray is Gamma rays often accompany the release of alpha or beta particles during radioactive decay. Gamma radiation is highly penetrating—no amount of absorbent material will completely stop or block gamma rays. A thick block of a heavy metal, such as lead, may be used to reduce exposure to gamma rays.

All types of nuclear radiation have one thing in common—they are all forms of ionizing radiation. The harmful effects of nuclear radiation are due to their ionizing ability, that is, their ability to strip electrons from atoms and produce ions when they travel through matter. The ionizing ability of different types of nuclear radiation is inversely related to their "penetrating power." Thus, alpha particles have low penetrating power but very high ionizing ability.

The cloud chamber was the first device for detecting ionizing radiation. Inspired by the beautiful colored lights observed when the sun shone on the clouds high in the Scottish hills, the Scottish physicist C. T. R. Wilson (1869–1959) built an apparatus to study these effects in the lab. In 1896, Wilson found that passing X-rays through a chamber containing supersaturated water vapor ionized the air in the chamber. Liquid droplets then condensed on the resulting ions, leaving visible “cloud tracks” to mark the path of the radiation. Later (1910–1913), Wilson also observed and analyzed the tracks produced by alpha and beta particles. Wilson received the Nobel Prize in Physics in 1927 for his method of detecting high-energy charged particles.

“Lantern mantles” provide a safe source of low-level nuclear radiation for use in the lab. Lantern mantles, which are sold for consumer use with portable gas camping lanterns, contain small amounts of thorium, a naturally occurring radioactive element. The Nuclear Regulatory Commission has studied the safety of lantern mantles and has estimated that a one-time camper would receive a radiation “dose” of about 0.06 millirems per year. (In comparison, the average individual in the United States is exposed to 1–2 millirems per year of radiation from watching television and an additional millirem per year from working in front of a computer.) Gas lantern mantles are a source of both alpha and beta radiation. Thorium-232, the primary radioactive isotope of thorium, emits alpha particles as it decays to generate radium-228, which then undergoes subsequent beta decay. See Equations 1 and 2.

Experiment Overview

The purpose of this experiment is to compare the “condensation trails” of alpha, beta and gamma radiation in a cloud chamber. The cloud chamber (see Figure 1) consists of a plastic container with lid, blotting paper and a radioactive source (the lantern mantle). Soaking the blotting paper with alcohol and then cooling the container on dry ice will cause the air in the chamber to become supersaturated with alcohol vapor. When the lantern mantle is inserted into the side of the chamber, radiation from the source will ionize air molecules in its path, and the resulting ions will cause alcohol droplets to condense on them and form tracks. The condensation trails will be observed by shining a flashlight into the chamber.

Materials

Prelab Questions

1. What kinds of materials can be used to shield or protect an object from alpha and beta radiation? What safety precautions are used in this experiment to protect against radiation released by the lantern mantle?
2. Complete the following table summarizing the properties of alpha, beta and gamma radiation. (See the Background section.)
   {12612_PreLab_Table_1}
3. Compare the penetrating power and the ionizing ability of alpha and beta particles. Which type of radiation should produce longer tracks in the cloud chamber? Which type of radiation will probably give thicker tracks due to the production of lots of ions?

Safety Precautions

A lantern mantle produces an extremely low level of radiation and is considered safe for consumer use. Wear gloves when handling the lantern mantle and wash hands thoroughly with soap and water afterwards. Do not light the mantle and avoid inhaling mantle dust. Isopropyl alcohol is slightly toxic by ingestion and inhalation and is a flammable solvent—do not use near flames or other sources of ignition. Dry ice is extremely cold and may cause frostbite. Wear insulated gloves when handling dry ice. Wear chemical splash goggles whenever working with chemicals, glassware or heat in the chemical laboratory. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Assemble the cloud chamber as shown in Figure 1 in the Overview.
2. Using a Beral-type pipet, soak the blotting paper liner inside the cloud chamber with 2 to 3 mL of 70% isopropyl alcohol. Add enough alcohol so that the blotting paper will be saturated—there should be only a thin layer of liquid in the bottom of the chamber.
3. Replace the lid on the cloud chamber and place a small cotton “plug” into the hole for the radioactive source (see Figure 1).
4. Place a solid block of dry ice or one large piece on a ceramic pad, then place the cloud chamber on top of the dry ice. Caution: Do not let dry ice touch unprotected skin.
5. Allow the cloud chamber to sit on the dry ice for approximately 5 minutes to cool the air and liquid and thus produce “supercooled” (supersaturated) alcohol vapor inside the chamber. (There will be a misty layer on the bottom of the cloud chamber, but the chamber should not cloud up completely.)
6. Insert the “puffy” side of the lantern mantle into the window on the side of the cloud chamber. The knotted end can remain outside the chamber.
7. Shine the light from a flashlight through the cloud chamber window opposite the radioactive source.
8. Observe the condensation trails against the dark, black bottom of the cloud chamber. Several different patterns of cloud tracks will be observed and they will change over time. Hint: The trails may look like wisps of smoke shooting out randomly through the chamber.
9. Observe the thickness, length and shape of the different kinds of tracks for about 5 minutes, and record all observations in the data table.
10. (Optional) Gently rub a piece of silk over the chamber lid. What happens to the tracks in the cloud chamber?
11. Return the lantern mantle to the instructor. Open the cloud chamber to dry out the blotting paper liner and allow the dry ice to evaporate in a well-ventilated area.

Student Worksheet PDF

12612_Student1.pdf