The Properties of Natural Radioactivity

Performance Objectives:

1. Explain what is meant by radioactivity and natural radioactivity.
2. Describe the types of nuclear radiation with respect to their mass, penetrating power, ionizing ability, nature and speed.
3. Demonstrate how to detect radioactivity
4. Determine what type of shielding is needed to stop radiation.
5. Demonstrate the mathematical relationship between cpm (count per minute) detected and distance [Content Standard Unifying Concepts- Change, constancy, and measurement]

Vocabulary:

Radioactivity is the emission of particles or electromagnetic energy from the nucleus of an atom.
Isotopes of elements are identified by the number of their protons and neutrons [9-12 Content Standard B- Structure of atoms]
Ionization power is the ability of the atom to form ions

Natural radioactivity is the radioactive decay of naturally occurring elements (elements with atomic numbers 1-92).

Materials:

Electroscope, rubber or plastic rod, a piece of fur or wool, pair of tongs, packaged alpha radiation source

Geiger counter; classroom-safe sources of radioactivity (alpha, and beta and/or gamma emitters); ruler; thin sheets of paper, of cardboard, of aluminum, and of lead, cut into small squares roughly 6cm x 6 cm

Advance preparation: cut the paper, cardboard and metal sheets into the 6-cm x 6-cm squares.

Safety: Use only radioactive sources that are classroom-safe and designed for the purpose and follow the directions that accompany them. [Teaching Standard D- Ensure a safe working environment] [Teaching Standard D- Make accessible science materials]

Motivation:

Start a lesson by reading this story to your students.

BRAZILIAN GOVERNMENT DEALS WITH THE WORST RADIATION ACCIDENT IN THE WESTERN HEMISPHERE (a true story)

On September 13, 1987, a stainless steel cylinder was removed from a cancer therapy machine located in an abandoned clinic in Goiania, Brazil. The two young scavengers who removed the cylinder, which was about the size of a 1-gallon paint can, sold it to a local scrap dealer for $25.

At the junkyard, the cylinder was dismantled, revealing a platinum capsule. Once opened, the junkyard employees observed a luminescent blue substance that resembled a salt. The glowing power was cesium-137, a highly radioactive substance; however, no one had knowledge of this fact at the time. Thinking that the substance was similar to carnival glitter, several people took the magical powder home. Reports said that some children rubbed the powder all over their bodies. One man slept with the powder under his bed and another carried it around in his pocket. A woman slept in clothing that was dusted with the powder.
On September 28, the scrap dealer went to a public health clinic complaining of severe vomiting and blistered hands and skin. Radiation sickness was correctly diagnosed. In the days that followed, 244 persons were found to have been contaminated, and 54 required hospitalization for test and treatment. Four people subsequently died and more than a dozen were seriously ill. The eventual toll of the accident will include hundreds of increased cases of cancer and genetic defects.

To safeguard the health of the public in this central Brazilian city of 1 million, buildings were washed down and radioactive soil was removed and placed in concrete-lined drums. Other states refused to buy grains, milk, meat, and vegetables from the state of Goias, in which the city of Goiania is located. Some citizens tried to block the burial of the four victims in the Goiania graveyard, fearing contamination of water supplies. Helicopters scoured the area with radiation detectors looking for hot spots. They identified contaminated furniture, cars, buses, money, and five pigs. Several houses and buildings were found to contain dangerously high levels of radioactivity.

Meanwhile, determining who was to blame for leaving the cancer therapy machine unattended for two years became a problem. The clinic owners, who abandoned the machine, as well as Brazil's Nuclear Energy Commission, the Ministry of Health and the Ministry of Labor, all shared some responsibility for maintaining, operating, and inspecting the machine. What will happen in the future is of prime concern to the citizens of Goiania. [9-12 Content Standard F- Natural and human induced hazards]

Strategies:

After reading the article above, ask them to identify the problem in the story and describe its cause and effect to the Brazilian City. Have students propose and implement solutions to the problem identified .

Ask students what comes into their mind when they hear the word radioactivity. Write their answers on the board. Then explain and write the definition of radioactivity and natural radioactivity on the board.

Explain to students that nuclear disintegration of naturally occurring radioactive atoms produces alpha particles, beta particles, and gamma radiation. These emissions differ from each other with respect to mass, charge, penetrating power and ionizing power.

To ensure interactive discussion, make sure that a reading material on the properties of the three types of particles is assigned as a homework before presenting this lesson to your students.

Assign students to groups. Ask them to create a chart showing the comparison of the three particles using a reading a material or reference on this topic. Have them present and explain their chart in the class. Discuss with your students the definition of nuclide and ionization power.

Teacher's Data Chart:

Comparison of alpha and beta particles and gamma rays

Particles Nuclide Symbols Mass
Number
(amu) Charge Penetrating power Ionizing power Speed Nature
Alpha 4He
2 a 4 + Relatively weak
(can be stopped by a single sheet of paper) Will ionize gas molecules About 1/10 the speed of light Sometimes behave like particles ;sometimes likes waves
Beta(electron) 0 e
-1 b- 0.00055 _ Greater than alpha(can be stopped by a thin sheet of aluminum) Will ionize gas molecules Approaching the speed of light Same as alpha particle
Gamma radiation g 0 0 Very penetrating (several centimeters of lead needed to stop them) Will ionize the atoms in flesh, causing severe damage to the cells Speed of light Electromagnetic waves of extremely short wavelength

Discuss to students how radioactivity can be detected by using the demonstration below.

Demonstration
Purpose: to detect radioactivity with an electroscope [9-12 Content Standard B- Properties of matter]

Procedure: 1. Ask a physics teacher for an electroscope, runner or plastic rod, and a piece of fur or wool. Rub the rod with the fur, then touch the top of electroscope.

2. Have the students observe the foil leaves of the electroscope. Then bring the source close to the electroscope.

CAUTION: Use only packaged radioactive materials. Handle with tongs. Have students observe the motion of the leaves.

Results: The two metallic leaves will spread apart as each is charged with electrons and they mutually repel one another. The alpha particles remove the electrons and the leaves fall back together.

Disposal: Place the substance in a plastic container and seal it. Completely label the container and, following proper storage directions, save it future activities.

Questions: 1. Are radioactive particles visible? No

2. Are alpha particles charged? Yes

Teacher Guide To Student Activity:

Using cooperative learning, assign a group to your students to investigate this activity.

Title: Distance, Shielding, and Radiation

Use this activity to show some properties of the different kinds of radiation

Overview: In this demonstration, the counts per minute (cpm) is recorded for radioactive source at various distances from a Geiger-counter tube to show a mathematical relationship between cpm detected and distance. [9-12 Content Standard E- Understandings about science and technology] [Content Standard Unifying Concepts- Change, constancy, and measurement] The type of shielding is needed is determined to stop alpha radiation and beta and/or gamma radiation.

Procedure: 1. Find the background count over a 2-minute interval. Record and repeat this process two or three more times, and record the average cpm. 2. Place an alpha emitter about 4 cm from the Geiger counter tube for 2 min., and have students record the resulting cpm, after first subtracting the average background count. Have students plot cpm(y-axis) versus distance of source from the Geiger-counter tube (x-axis). Ask them to describe the resulting curve. [9-12 Content Standard A- Use mathematics to improve scientific communication] 4. Place an alpha emitter 2-cm from the tube, place 1 sheet of paper between the source and the tube, and find the average background corrected cpm. Repeat keeping the distance constant and using more sheets of paper until the cpm is reduced essentially to the background count. Repeat the same procedure, using sheets of cardboard, then sheets of aluminum, and, finally, sheets of lead. Have students compare the materials' abilities to act as shields against alpha particles. 6. Repeat step 4, suing a beta and/or gamma emitter.

Results: The curve produced in step 4 should illustrate an inverse square relationship of the general shape shown in accompanying Figure 26-1 (vertical-axis units will depend upon the particular source used.) In step 5, alpha radiation essentially will be stopped by a few sheets of cardboard, relatively few sheets of paper, 1 sheet of lead, and 1-2 sheets of aluminum. Beta and/or gamma radiation will not be affected significantly by the paper; a relatively large number of sheets of aluminum or several sheets of lead will be required to essentially stop the radiation.

Summary:
1. Use the demonstration above to summarize the lesson, if time permits.
2. Have students list the three types of radiation and their properties on a piece of paper.

Enrichment:

1. Have students research how radioactive particles affect the chemicals in photographic emulsions and how this effect is used in medicine.
2. Write a brief report on the scientific contributions of Henri Becquerel, Marie and Pierre Curie. Describe the history of their work, their stories and their roles in the development of nuclear science. [9-12 Content Standard G- Historical perspectives]

Regents-Type Questions:

1. Which particle is electrically neutral?
(1) proton (2) positron (3) neutron (4) electron'

2. The atoms of some particles can be made radioactive by
(a) placing them in a magnetic field
(b) bombarding them with high-energy particles
(c) separating them into their isotopes
(d) heating them to a very high temperature

3. Which particle has the same mass as an electron, but positive electric charge?
(a) Alpha particle (b) gamma photon
( c) gamma photon (d) positron

4. When a stream of radioactive particles is passed through a pair of oppositely charged parallel plates, which particle would be deflected toward the negative plate?
(a) alpha particle (b) beta (-) particle (c) Gamma radiation (d) neutron

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