Cloud Chamber Lab

Astronomy in the Ice Cloud Chamber Activity "Cosmic Ray Contrails"

Overview:

Abstract: You will construct a Cloud Chamber, which is a cosmic ray detector. Every second, high-energy particles are passing right through your body! Physicists study cosmic rays (high speed protons, electrons etc.) and other particles indirectly by detecting effects of the particles as they pass through detectors. The particles either collide with or otherwise affect the molecules making up the detector. These atoms or molecules then break into pieces, give off light, or in this case condense to form clouds. In your cloud chamber high energy particles, formed as the result of cosmic ray collisions high in the atmosphere, will leave a condensed trail of alcohol, just as a high flying jet plane leaves a trail of water crystals. Your cloud chamber should produce similar trails as particles are detected.

Rationale: Scientists study cosmic rays and other high speed particles in order to understand which particles make up matter, how they interact, and how much energy they have. By learning how energetic they are and where they are produced (atomic reactions, stars, black holes, super novas) physicists uncover the past, explain the present and predict the future.

Most high-energy detectors cost millions, take years to construct and additional years to complete an experiment. Your cloud chambers should only take a period to construct and at most an additional class period to use. You will observe the evidence which originally led to the discovery of cosmic rays!

Grade Level/Discipline: Appropriate for sr. high physics and/or chemistry. Modifications to pre- and post lab questions as well as additional supervision to address safety concerns would be required for middle school application.

Objectives: Student will:
Construct a cloud chamber
Observe the effects of high energy particles (concrete experience vs. theoretical awareness)
Observe and explain the condensation of vapor
Describe and define saturation, ionization, and condensation

1
Connection to curriculum/ Teacher preparation for activity

Standards:

Information for the standards came from the "WISCONSIN MODEL ACADEMIC STANDARDS FOR SCIENCE" Web site at http://www.dpi.state.wi.us/dpi/standards/sciintro.html

A.12.3 Give examples that show* how partial systems*, models*, and explanations* are used to give
solutions that are accurate enough for basic needs

A.12.4 Construct* arguments that show* how conflicting models* and explanations* of events can
start with similar evidence*

B.12.5 Explain* how science is based on assumptions about the natural world and themes* that
describe the natural world

C.12.1 When studying science content, ask questions suggested by current social issues, scientific
literature, and observations* of phenomena, build hypotheses that might answer some of these
questions, design possible investigations*, and describe results that might emerge from such
investigations

C.12.2 Identify* issues from an area of science study, write questions that could be investigated*,
review previous research on these questions, and design and conduct responsible and safe
investigations to help answer the questions

C.12.3 Evaluate* the data collected during an investigation*, critique the data-collection procedures
and results, and suggest ways to make any needed improvements

C.12.4 During investigations*, choose the best data-collection procedures and materials available,
use them competently, and calculate the degree of precision of the resulting data

C.12.7 Evaluate* articles and reports in the popular press, in scientific journals, on television, and on
the Internet, using criteria related to accuracy, degree of error, sampling, treatment of data, and other
standards of experimental design

D.12.1 Describe* atomic structure and the properties of atoms, molecules, and matter during physical
and chemical interactions*

D12.2 Explain* the forces that hold the atom together and illustrate* how nuclear interactions*
change the atom

D.12.3 Explain* exchanges of energy* in chemical interactions* and exchange of mass and energy in
atomic/nuclear reactions

D.12.4 Explain* how substances, both simple and complex, interact* with one another to produce
new substances

D.12.7 Qualitatively and quantitatively analyze* changes in the motion of objects and the forces that
act on them and represent analytical data both algebraically and graphically

D.12.8 Understand* the forces of gravitation, the electromagnetic force, intermolecular force, and
explain* their impact on the universal system

D.12.11 Using the science themes*, explain* common occurrences in the physical world

E. 12.1 Using the science themes*, distinguish between internal energies* (decay of radioactive
isotopes, gravity) and external energies (sun) in the earth's systems and show* how these sources of
energy have an impact on those systems

Materials List:

Materials for each group

Large plastic or glass jar with a metal lid. If you use plastic it should be #1 so it will not degrade by the ethanol.
Flat cover: Glass, Plexiglas or a clear #1 plastic.
Silicone: "aquarium adhesive"
Black color fast paper
Solid CO2 (dry ice)
98% pure ethanol
Black ink
Wax paper
Light source: 150W or better, a maglite flashlight works well
Radioactive source: optional

Pre-activity Set-up:

The facilitator should bring all of the materials to class before the lab beings. It should only take 30 minutes to construct the detector. If the facilitator is going to build the detector for the students a period of time will be need to construct the devices. Also because their detector will be reused the first group will take more time to build and set up. The dry ice and the 98% pure ethanol will need to be ordered or purchased before hand. This lab will take approximately two hours from set up to clean up. Students may need to rework their detector if problems occur, causing more delays.

Teaching Sequence:

Pre Lab Lecture/Discussion:
A. Introduce/Review ionization
a. What is an ion?
b. How are ions produced?
c. How do ions affect their surroundings?
d. Is there a difference between positive and negative particles?
i. How can they be identified?
B. Introduce/Review saturated solutions.
a. Modes of producing condensation
i. Temperature effects (e.g. water vapor condensing on glass of ice water)
ii. Particle effects (e.g. seeding a cloud)
C. Introduce/Review properties of ethanol as an evaporative liquid.
D. Discuss cosmic rays and properties
a. If a radioactive source is to be used to produce more consistent tracks review of alpha/beta decay would be necessary.
E. Review lab procedures/rules

Building the detector:
A. Refer to Classroom components section

Pre Lab Student Questions:

1. What are cosmic rays?

2. How are the cosmic rays we expect to see in our cloud chambers produced?

3. Give an example of a super-saturated liquid/vapor that you have encountered personally?

4. Explain temperature gradient in your own words. (Hint where would like to sit in a large auditorium on a 95 day for your cousins graduation ceremony?)

The Lab Experience:

Post Lab Student Questions:

1. What is the purpose of the dry ice (solid CO2) in cooling the chamber?

2. Why is sealing the cloud chamber important?

3. Why is ethanol used instead of water in the chamber?

4. Do the tracks travel in straight lines, broken lines, curves? Explain.

5. Do the tracks seem to originate from the same direction?

6. How is the contrail of a jet similar to the "tracks" we are seeing in the cloud chamber?

7. Would this detector be able to detect a neutron? Explain.

8. Do the trails differ in length or width? How might you account for the difference?

9. If cosmic rays move a very high speed, how are we able to see trails that move a slow speed?

10. Are cosmic rays able to penetrate buildings?

Draw and label a diagram of the experiment and the results you observed.

Above and Beyond: (for enhancement)

1. Measure the # of counts you get in 5 minutes. Estimate the # of counts you would get in 24 hours (Cts/day).

2. Repeat the experimental in #1 above count with a radioactive source. How many of the visible counts are produced by the source? e.g. how do we factor out the cosmic ray background?

3. Research the difference between alpha and beta sources. How would you alter the experiment to separate these effects?

4. Would you see a different count rate in the original experiment if you were in the space shuttle? On the moon? On Mars?
Explain.

5. What improvement in design would you recommend for better results?

6. Summarize the experiment as you would to a student who has missed the last 3 days of class?

Student Assessment

Explanation

What do the trails in the cloud chamber represent? (Misconception Potential) Student might think that they are seeing the particle rather than the interaction. Students are familiar with contrails left in the sky overhead by jet aircraft. A contrail may be visible without someone being unable to see the aircraft. The trail left by the radiation is similar. A cloud chamber is full of super-cooled vapor looking for somewhere to condense. Water droplets condense on the trail of ions left by a charged particle traveling through the chamber.

What role does the ethanol play? Students are familiar with their ability to see suspended small particles in the air when they look back toward the projection booth in the movie theater. Ethanol has a low evaporation pressure. Above the dry ice a layer of saturated air is created. As the particle passes through the air it ionizes the air. The polar molecules condense in the ionized air creating the vapor trail seen via the reflected light.

Exchange

How do the observed tracks differ from each other?

What causes the observed differences?

What are the sources for the particles that caused the observed trails?

How would a magnet near the chamber affect the path of the particle?

Evaluation

Write a paragraph that explains the formation of the particle trail observed in the cloud chamber.

Classroom Components:

Background:
The teacher should have a good background in cosmic rays and how they produce the clouds witnessed in the chamber.

Misconceptions:
Students may think the clouds are cosmic rays.
Students may not correctly understand the origin of the clouds.

Resources and References:
http://www.scidiv.bcc.ctc.edu/Physics/Cloudchmbr/htm (Cloud Chamber Description)
http://www.cloudchambers.com// (Commercial Cloud Chamber site to steal ideas from)
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html (Powers of Ten/Size-scale site)
http://www.exploratorium.edu/snacks/fog_chamber.html (Other designs)
http://www.infoplease.com/ce6/sci/AO812625.html (Description of Reactions)
http://www.freeweb.pdq.net/headstrong/cloud.htm (Description of Reactions)

General Activity Description (Procedure):
1. Cut the pickle jar a vertical height of 10 cm from the metal lid. The cut should be made parallel to the flat surface of the lid. 2. Apply the silicone adhesive to the cut edge of the jar. Place this edge of the jar on a flat surface lined with wax paper to dry.
2. Cut the black paper to the appropriate size so that it will line the edge of the jar. Leave a 2.5 cm narrow slot on the side of the jar. You may a second strip if you wish.
3. Cut a hole in the top of the Styrofoam cooler the diameter of the lid of the jar.
4. Cut the cooler to a height where this lid can fit upside on the cooler.
5. Wear gloves before touching the dry ice. Place the dry ice in the container.
6. Set jar upside down on the ice so that the metal lid is touching the surface of the ice.
7. Place the paper inside the jar. If you do not use ink to color the bottom black, cut paper to cover the lid.
8. Soak the paper with ethanol and leave a shallow pool in the bottom of the chamber. Add ink if you haven't put black construction paper on the bottom of the chamber.
9. Turn the lights off in the room. Position the light source so that you can see the cosmic rays (for a smaller light source try a shallow angle-a flashlight with a wider beam will probably work better by shining it directly down into the chamber). Look for naturally occurring radiation in the room.

Analysis:
1. What are some important things you need to have happen so that your cloud chamber functions properly?
2. Describe the appearance you will see in the cloud chamber when cosmic rays pass through.