Physics 318
Spring 2010
Name(s): ____________________________________
Experiment 1: Cloud Chamber Tracks
Objective: To set up a cloud chamber and classify the various types of cosmic ray tracks created inside,
and investigate the effects of a strong magnetic field.
Background: A diffusion cloud chamber is a device used to detect ionizing particles. Alcohol is
introduced into the top of the sealed container and the bottom of the container is cooled by
means of dry ice or liquid nitrogen. The alcohol vapors diffuse toward the bottom of the
container forming a layer of supersaturation at the base which appears as a hazy cloud.
Supersaturation means that the alcohol is in a vapor at a temperature below which it would
usually be a liquid. The vapor molecules condense along the ionized path made by the ionizing
radiation resulting in a visible trail or track of droplets of alcohol.
Cloud chambers were first built by Charles Thomson Rees Wilson in the 1911 when he
was attempting to recreate natural clouds to study could formation. The expansion type cloud
chamber built by Wilson used water vapor and an adiabatic expansion process. Wilson also
found that when ionized particles such as electrons passed though his cloud chamber, they would
leave a trail of condensation behind them. Soon after this discovery, positrons and muons were
discovered by others using this same type of cloud chamber.
An electron or beta particle passing though the supersaturated environment leaves a thin
white track. These tracks can be straight lines or they may have a sharp turn or even many turns
creating a zigzag pattern. These are from collisions with other particles in the chamber (or may
be from a muon decaying into the electron). The particles may also spiral in a magnetic field
depending on the speed of the particle and the strength of the field. Protons and alpha particles
appear as slower developing, thicker tracks. They also have an electric charge, so their path is
modified by a magnetic field.
The distance the ionizing radiation can travel in a material is related to the energy of the
ionizing radiation. The length of the track in the cloud chamber can be used to estimate the
original energy of the incident radiation. Graphs of range vs. energy for electrons (Figure 1) and
alpha particles (Figure 2) are listed below. REFERENCE NEEDED
Materials:
 Cloud chamber with base
 Rare Earth Magnet (BE CAREFUL, these are extremely strong!!!)
 Ethyl alcohol in a squirt bottle
 1 slab of dry ice
 Webcam w/ USB cable
 Focused light source
 Radioactive sources
- 1 alpha particle emitting source
- 1 gamma ray emitting source
Figure 1:
Figure 2:
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Procedure:
1. With gloves, center the slab of dry ice on the base. Remove the cloud chamber’s cover and place
the chamber directly on top on the dry ice. Make sure the metal base is in direct contact with the
dry ice. Soak the felt in the tank with alcohol until alcohol begins to drip down the sides from
the felt. Place the cover back on the chamber and place your light source at the base of the cloud
chamber shining through about 12 inches away. Arrange the webcam so that it has a good view
of the interior ‘sweetspot’ near the bottom of the chamber. On your laptop, setup the camera
software so that the camera saves to a file. Now you must wait for the saturation level to appear.
This usually takes around 15 minutes but it may take up to 30 minutes for the environment inside
the chamber to become ideal.
2. When you begin seeing prominent tracks, record the tracks on the webcam for about 5 minutes.
Watch the video you just recorded looking for the longest alpha and electron tracks. Measure the
length of the tracks and use the Range-Energy graphs to find the approximate energy of the
longest electron track and alpha particle track. Now go back to the beginning of the recording
and recount every particle that you can see to get a background count rate (particles per minute).
3. Remove the webcam, open the cloud chamber and place an alpha emitting radioactive source in
the cloud chamber. When you begin seeing significant tracks from the alpha source, record via
webcam for about 5 minutes. Watch the video you just recorded, and estimate the length of the
longest alpha track. Use Figure 2 to find the corresponding energy of the longest track.
4. Remove the webcam and take the cloud chamber off its base. Place a magnet under the tank,
nestled in the dry ice. Replace the tank and wait for the supersaturation layer to reappear. Once
it does, look for the tracks that are curling in different directions. If you look long enough you
may see two electron type tracks coming from the same location but curling in different
directions. You do not have to record anything from this step, since curling is difficult to see
conclusively, but I would love for someone to catch a nice particle anti-particle pair creation
event on video.
5. If time permits, open the chamber again to replace the alpha source with a high energy gamma
ray source in the tank. Do you notice a significant increase in the event rate compared to the
background count rate measured in part 2?
Group Questions: (Answer on separate sheet)
1. What was the longest electron track you recorded and what was its corresponding energy?
2. What was the longest background alpha particle track you recorded and what was its
corresponding energy?
3. What was the length of the alpha tracks from the alpha source and their corresponding energy?
How does this estimate compare to the alpha’s energy expected to be emitted by this source?
4. What was your background count rate?
5. Did you see any tracks that curled in opposing directions near the magnet? What could it mean
if electron-like tracks curl in opposite directions? Should they behave identically in the same
magnetic field? Even if you didn’t see this, try to explain it.
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References:
1. Radiological Health Handbook, U.S. Department of Health, Education and Welfare, Washington
D.C., 1970. pp 123-125.
2. Glenn Knoll, Radiation Detection and Measurement 2nd Ed. John Wiley and Sons, New York,
1989.
3. National Institute of Science and Technology – Physics Technology.
http://physics.nist.gov/PhysRefData/Star/Text/contents.html Accessed on July, 2007.
4.
Kenneth Krane, Introductory Nuclear Physics. John Wiley and Sons, New York, 1988.
5. Diffusion Cloud Chambers Owner’s Guide, Version 2.5. Reflection Imaging, Inc. 2001.
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