Download DanMarkup2009.01.13CloudChamber - Physics

Rich, I'm editing this as if it were a PHY690 only manuscript. I encourage you to re-use it
for many things (though you'll have to be careful about self-plaigerizing if you do publish
it). However, I'm marking it up for PHY690 appropriateness here
Constructing and teaching with an
Electrically Cooled Cloud Chamber for the
HS Physics Classroom
Richard Gearns, Department of Physics, SUNY-Buffalo State College, 1300
Elmwood Ave Buffalo NY 14222 <[email protected]>
Put all figures on single pages at end of ms (after refs) and put the text "INSERT
Fig nn About here" in the ms please,
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Abstract
To introduce high school students to the world of particle physics, teachers can
build a large electrically cooled cloud chamber for under $1000 using off the
shelf components and easily sourced materials. This paper describes the
construction of an electrically cloud chamber and its uses in the classroom. An
electrically cooled chamber permits flexibility of scheduling its use and eliminates
the need for purchasing, storing, and handling dry ice or liquid nitrogen. The
chamber design incorporates a magnetic field to facilitate estimation of the
momentum of observed particles. The large size of the design allows students to
explore natural background cosmic rays as well as radiation from a variety of low
cost radioactive source materials like radioactive sample disks, Fiestaware, and
NoSalt.
Did you mean NuSalt? Have you ever actually tried NuSalt or a banana in your
cloud chamber? Just curious. NuSalt is supposed to taste quite gross.
C.f. < http://www.orau.org/PTP/collection/consumer%20products/consumer.htm>
Acknowledgement This needs to be less than a para TOTAL See suggested:
This manuscript was produced in partial fulfillment of PHY690: Masters' Project
At SUNY-Buffalo State University Physics under the direction of Dr. Dan
MacIsaac. Portions were supported by the US Dept of Energy Quarknet
program. Dr.Helio Takai and Augie Hoffman of Brookhaven National Laboratory
provided guidance, patience and support.
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By its nature, a project of this type incorporates the advice and help of many
people. Thank you to the Fall 2005 students of PHY 579 what school? for the
initial inspiration for this project. The hours of setup and breakdown of the dry ice
cloud chamber and the initial tests of the thermoelectric cooler concept lead to
further trials and ultimately the current design.
A debt of gratitude goes to Brookhaven National Laboratory technician Augie
Hoffman. Augie’s remarkable ability to create exactly what you need even when
you have trouble describing it to him, made building the chamber possible and
repairing the chamber after various mishaps bearable.
Thank you to the Department of Energy’s QuarkNet program for providing
funding for all of the components and materials used in the development of the
chamber.
Significant appreciation goes to Daniel MacIsaac PhD for critiquing this
manuscript and providing sound suggestions toward making a succinct
presentation of this project.
Special thanks goes to Helio Takai PhD. His willingness to have coffee at any
hour to answer questions will never be forgotten. No matter the question, Dr.
Takai has the unique ability to make you feel intelligent, even if the answer
should be obvious.
Introduction
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Alexander Langsdorf invented the diffusion cloud chamber in 1938 using
methanol vapor diffusing through carbon dioxide gas in a six-inch diameter
cylinder (Langsdorf, 1939). The original diffusion chamber never enjoyed
widespread use among the physics community of the 1940's and 50's (Nielsen,
Needles, & Weddle, 1951). However, cloud chambers have enjoyed a
resurgence in science education. While considered ancient technology by today's
research standards, these tools are easily accessible to teachers and students
and provide a direct, hands-on means of introducing particle physics to students.
Homemade simple and inexpensive chambers that provide a view of cosmic rays
can be made from Pyrex lantern globes, cotton wicking, and metal candy
containers (Ketner, 1965). Several commercial classroom demonstration
chambers exploit new electronic cooling mechanisms, but small size and
construction materials limit their usefulness. For example, the chamber available
from Pasco (www.pasco.com) and other suppliers have a small 15 cm diameter
viewing window on top and opaque sides. As a result, students find it difficult to
see long sweeping tracks. Few, if any, classroom models incorporate a magnetic
field in their design. Identification of particle types and estimation of their
momentum requires the presence of a known magnetic field. Many larger
designs, like those from Supersaturated Environments
(www.cloudchambers.com) prescribe the use of large amounts of dry ice or liquid
nitrogen to achieve the low temperatures necessary for establishing the
supersaturated environment needed to see cosmic rays. Purchasing, handling,
and using dry ice or liquid nitrogen in the secondary school setting presents
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several challenges discouraging use in the classroom. A large, electrically
cooled, diffusion cloud chamber with magnetic field provides significant
opportunities for exploring particle physics because of the flexibility it provides in
scheduling and the capability to do real science in the classroom.
A single para (and more importantly references) on HOW a cloud chamber works
would be most appropriate and welcome. And this preceding para can still be cut
down (you repeat yourself between start and end).
Construction
The design centers on a 14" x 8" x ¼" black anodized aluminum plate. Aluminum
may be sourced from a variety of suppliers, including McMaster-Carr
(www.mcmaster.com), cut to size, and sent to a metal finisher for anodization.
Cost? Black hard anodization provides the best combination of contrast and
durability. See Fig 1
Commercial thermoelectric coolers chill the chamber (Fig 2). This technology
uses the Peltier Effect where current passing across the junction between
dissimilar metals causes an overall decrease in the temperature of the junction
(Deeson, 2007). Four Thermal Enterprises LLC model CP-12726 thermoelectric
coolers positioned beneath each of the four corners of the plate pump heat from
the chamber to water filled heat exchangers. Each cooler uses up to 25 amps of
current at 12 volts to achieve the minimum -25°C temperature necessary for
cloud formation. Each electrical unit (cooler?) forms the center of a sandwich
where the aluminum plate of the chamber contacts the cold side of the
thermoelectric unit and a water-cooled copper "chip cooler” or heat exchanger
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contacts the hot side. Four Alphacool model NexXxos HP Pro AMD "chip
coolers" fit precisely over the 5 cm x 5 cm surface of the thermoelectric elements.
A Hydor SELTZ L35 submersible pump circulates water through a system of 3/8"
vinyl tubing (Fig 3). The pump delivers 0°C water to each heat exchanger in
parallel and returns the warmer exhaust water back to the ice bath (housed in a
large camping cooler – size? supplier? Cost?) for re-chilling. While rated for 1700
liters per hour, the pump delivered an average of 219.4 liters per hour in this
configuration. In theory, the thermoelectric units provide a maximum temperature
differential of 70°C. In practice, the best achievable differential hovered around
35°C. Therefore, the circulating 0°C water provides the units with a low constant
base temperature below which which the thermoelectric units can further lower
the plate temperature.
Four Mean-Well model S-320-12 switching power supplies provide 12 volt
current to each of the four thermoelectric elements. Few power supplies can
supply the nearly 100 total amps of current at 12 volts for a reasonable cost.
Less expensive supplies, like car starters, can provide the large current
necessary, but only for a short time before overheating and shutting down. (Fig
4) During initial trials, the supplies were independently connected to the mains.
For safety and ease of setup, the four power supply units were stacked in a
custom aluminum rack and fitted into an old tower computer case. All four units
connect in parallel to a 20-amp ground fault interrupter and then on to a 15-amp
power cord. Power draw peaks at 1147 watts as measured at the outlet with a P-
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4400 Kill-A-Watt meter (cost? supplier? Harbor Frieght??). Ten gauge stranded
wire connects each supply to its corresponding thermoelectric unit. Using
independent supply/cooler pairs allows easier adjustment and troubleshooting.
Integral to the design is 1/8" thick clear Lexan (plexiglas? – supplier? Cost??)
fabricated into a 14" x 8" x 7” "tank" that fits snuggly over the aluminum base.
Modeled after an inverted fish tank, observers can view from all sides and the
top. The cover completely encloses the aluminum cooling plate. Gluing the
smooth edges of the five panels together with Locktite (italicize all commercial
company names and trademarks please) eliminates the black frame and silicone
adhesive typical of aquariums and provides better viewing from more angles. The
tight fit helps eliminate air leaks that may disrupt the stable environment required
for precipitation of the ethyl alcohol. Drilling a small hole in the cover provides a
means of injecting more ethyl alcohol, if necessary, while chamber is operating.
The magnetic field for the chamber comes from an Applied Magnetics 4 inch x 6
inch x 1 inch C8 ceramic magnet imbedded in the Styrofoam insulation and
centered below the aluminum plate between the thermoelectric coolers. The
poles exist on the largest flat surfaces with the North pole directed upward
through the bottom of the aluminum cooling plate. The magnetic field strength
peaks at about 950 gauss at the center of the plate surface as shown in figure 1.
The plate and thermoelectric cooling assembly rest on a 2" bed of foam
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polystyrene insulation available from Lowe’s or Home Depot with cutouts for the
thermoelectric elements and water blocks. One additional 4" x 6" cutout was
made for the magnet directly under the middle of the plate. The top of the magnet
must be insulated from the aluminum plate to reduce the cooling load on the
thermoelectric units. Both the insulation and plate nest into an 18” 12” x 10”
wooden box. Electrical connections are made from below using 10-gauge wire
and wire nuts. Water connections are also made from below using 1/4" to 1/2"
hose clamps.
Operation write this is past tense
Operation requires approximately 50lbs of ice, 100mL ethyl alcohol (100% details
suppliers cost), 1 meter of Mortite rope caulk, and a quality, streak-free glass
cleaner. Chamber set-up requires a dark room with space for a (how large?)
redundant with earlier?? large camping cooler to be used for the ice bath. A
nearby sink or drain aids setup and breakdown excellent point. Pouring water in
the ice bath helps prime the pump submerged in the bottom of the ice bath.
Keeping the ice chest at the same level as the chamber reduces the head
pressure on the pump thereby increasing the water flow through the cooling
units. Place the power supply unit next to the chamber. Take care to place the
power supplies safely away from water leaks. Also, keep potential sparks away
from the ethyl alcohol.
Next, clean the plastic enclosure of fingerprints and streaks using a lint free cloth
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and the window cleaner. Add 100% ethyl alcohol to the (how much felt padding
is used where) felt padding by soaking thoroughly. Put a small, clear plastic ruler
inside to provide fiducial marks. Next, fit the enclosure over the aluminum plate
and seal to the polystyrene insulation with Mortite. Set up an overhead projector
by narrowing the projector beam to a thin horizontal line using manila folders on
the surface of the projector and directing the beam to the far side of the chamber.
A proper beam slopes gently downward toward the opposite corner where the
plastic enclosure meets the aluminum plate.
Start the pump. Use the first two minutes to check for leaks and bleed air from
the hose system. Briefly plugging the exhaust hose and releasing bleeds air out
of the system. While the plate temperature falls as the 0°C water circulates
through the system, switch on the thermoelectric coolers. Do you have a
termometer anywhere? Recommend a non contact IR thermometer from Harbor
Freight?? Cost? The plate temperature will fall rapidly toward operating
temperature. Because there are no thermoelectric chillers in the middle of the
plate, the outside edges chill more rapidly than the middle. Turn out the lights
and turn on the overhead projector.
In fifteen to twenty minutes, the supersaturated alcohol vapor forms what?? a
cloud?. Supersaturation occurs when the alcohol evaporating from the warmer
top of the chamber condenses near the cooler bottom of the chamber. This is
evidenced by tiny alcohol droplets forming about 1 to 2 cm above the surface of
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the plate and then falling toward the plate as an Alcohol rain. Observation of
cosmic ray events occur only when the plate temperature gets cooler than -25°C
for initial events that occur at plate locations directly over the thermoelectric
coolers. Although some areas of the plate reach -25°C within six to seven
minutes, It will take up to 20 minutes for the proper temperature gradient to set
up and for cosmic rays to appear in the center of the plate where the magnet is
located. Describe the cloud mechanism in your intro please.
The longevity of the chamber depends on the amount of ice in the chest, the
temperature gradient, and the concentration of alcohol in the enclosure. The
optimal temperature gradient from the top of the chamber to the bottom plate is
6°C/cm (Alston, Crewe, & Evans, 1954). If the droplets become sparse and
events diminish or stop altogether, the temperature gradient may not be large
enough and may be improved by gently warming the top of the enclosure with
your hand or a 5 to 10 watt reptile heating pad (cost?). If this doesn't improve
matters, alcohol can be added through the small port in the top of the enclosure.
In either case events should resume within several minutes. Additional ice can
always be added to the ice chest without interrupting operation of the chamber.
When finished using the Cloud Chamber, you must carefully sequence
shutdown of the system to avoid water freeze damage to the coolers.
Another sentence or two on this is appropriate.
Classroom Activities
This chamber design is optimized for observing cosmic rays . However, before
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the cosmic ray paths become visible, students will see the small droplets of
alcohol rain begin falling toward the cold plate as the temperature gradient sets
up, and will soon see sweeping arcs of charged particles. When the trails
become visible, students can sketch all the different paths that they see. Some
paths will be straight while others will curve to the left or right. Others will
resemble corkscrews. Still others crash their way through the cloud seemingly
bumping into things and changing directions. Some example images produced
with this chamber may be viewed at: http://wwwmariachi.physics.sunysb.edu/wiki/index.php/CCI. Some other sites explaining
and reviewing particle tracks include:
plasma.org/activity/education/Projects/Teaching%20Advanced%20Physics/Atomi
c%20and%20Nuclei/.../file_5083.doc
etc etc
Students should note the inconsistent radii of the curved paths. Once students
have seen the chamber up close, training a webcam or camcorder on the active
region and projecting the image to a television or large movie screen using an
LCD projector goes a long way toward keeping and focusing the attention of the
class by allowing all of the students to see and discuss the same live image.
Recording the images on video allows the class to search for, replay, study, and
research interesting events making video a powerful tool to use with the chamber
(Cassidy, 1994). Some of the most interesting tracks students will observe in this
chamber are low momentum tracks. This is due largely to the relatively low
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magnetic field strength of this chamber. The availability of some very large rare
earth magnets with three to four times the magnetic field strength of the ceramic
magnet make upgrading the magnet now built into this chamber an attractive
option. However, the ability to safely handle these powerful magnets must be
deliberately considered before going further. Tighten or eliminate this last.
Low momentum tracks produced by low momentum muons and electrons are
easily deflected by the magnetic field. Measuring the particle's radius of curvature
using fiducial marks in a known magnetic field allows estimation of momentum.
Tracks of electrons, for example, loose energy as they interact with nuclei in the
media through brehmsstrahlung radiation. As the particle interacts with the
electric field of the nucleus it may emit photons, electrons, or positrons as the
radius of curvature of the original particle becomes smaller. Photons will not
produce additional tracks. However, positrons and electrons both produce tracks
that will turn in opposite directions within a magnetic field (Veltman, 2003). Show
examples or cite photos (see
plasma.org/activity/education/Projects/Teaching%20Advanced%20Physics/Atomi
c%20and%20Nuclei/.../file_5083.doc and find more)
Particle decay is also interesting. At sea level, where this chamber has been
developed and operated, it is rare. Most decays are muons decaying into
electrons. Again the magnetic field provides a nice way for students to examine
these events, because it provides a way to identify tracks. Electrons, because of
their light mass, scatter easily in the cloud, whereas muons, which are 200 times
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more massive than electrons, (Bunce, 1996) do not scatter as much.
Students may recognize Delta rays, or electrons scattered out of their orbits, by
looking along the tracks. As a high-energy particle blasts through, small branches
form. The branches are orbital electrons knocked out by Rutherford scattering.
Pair production is perhaps the most interesting phenomenon seen and provides
direct observation of Einstein's E=mc² relationship. High-energy photons passing
through the electric field of a nucleus can create a positron-electron pair
(Veltman, 2003). Since both the positron and the electron have identical
momenta, but opposite charge, their paths will have the same radius with
opposite directions. Students may be guided with appropriate questions to help
them recognize traits that reveal a particle’s identity. For example, in the case of
pair production, the teacher may ask, “How do we know there was a gamma ray
(Lehrman, 1968)?” Online photos and examples??
Mapping the magnetic field of the chamber makes an estimate of the particle's
momentum possible. This is easily accomplished with a "Cheap Gaussmeter"
(MARIACHI Project, 2006) made for under $10.00. Because of the relatively thin
(1 to 2 cm thick) active cloud region and the fact that the exact trajectory of a
cosmic ray trail can not be exactly determined in three dimensional space using a
single camera, the magnetic field map only needs to give detail for the region
immediately near the surface of the aluminum plate. While high-energy particles
will be observed in the chamber, their radius of curvature will be very large; the
momentum cannot be measured in this chamber. Only low energy events will
display easily identifiable curvature. The best events for measuring the
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momentum should be where the path is parallel to the plane of the aluminum
surface. When an interesting event has been identified in a video, a student can
pause the recording and use overlay software like that that provided with
SmartBoards (www.smarttech.com). They can use the program to create a circle
that matches the radius of the particle's trajectory and then move the circle over
the fiducial in the same field of view to get a measurement. Alternatively, still
pictures can be inserted into Microsoft Word and a circle from the shapes menu
matched up to the path of the particle. Since the momentum of the particle
generally decreases, the path radius diminishes slightly offering an opportunity to
discuss the momentum lost by the particle. Students use introductory physics
concepts to analyze the event. Hand rules will help the students identify the
charge of the particles. As the majority of particles are muons, the students can
assume the charge of the particles to be e (Schuh & Barradas, 2003). The
momentum calculation is straightforward:
r
r r
F  qv  B 
mv2
F
 qv B 
r
p  qBr
Tighten this last up and show some sample figures.
In addition to thecosmic ray background radiation, students can explore radiation
from other radioactive sources in the chamber. “Fiestaware,” camping lantern
mantles, No Salt salt substitute, are easily obtainable (Nave, 2001) radioactive
sources sold on EBay, camping outfitters, and groceries respectively. Students
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can wrap these sources in a variety of materials like paper or aluminum foil to
investigate the effect on the ionization trails (American Nuclear Society, 2001).
Conclusion
Although an ambitious project, building a large electrically cooled diffusion cloud
chamber makes introduction and study of subatomic particles fascinating for the
students using introductory physics concepts. The large size makes track
visualization easy, the magnetic field helps students see the impact the Lorentz
force has on charged particles, and the electric cooling system makes dry ice a
thing of the past. Overall, an electrically cooled cloud chamber is a big,
impressive, and satisfying project making the invisible world of cosmic rays
visible in your classroom. This Last sentence is gramatically incomplese
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Appendix:
Table one: Sourced materials and costs:
Qty
Brand
Model
Supplier
Cost
(each)
4
Mean-Well
S-320-12
TRC Electronics
$70.98
www.trcelectronics.com
4
Alphacool
NexXxos HP
Pro AMD
FrozenCPU.com
$34.95
www.frozencpu.com
CPU Block
4
Thermal
Enterprises
CP1-12726
Thermal Enterprises
$18.20
[email protected]
(732) 821-5301
1
1
P3
International
P-4400
Amazon.com
Kill-A-Watt
www.amazon.com
Hydor-SELTZ
L35
Blue Water Corals
$22.94
$64.99
www.bluewatercorals.com
1
Applied
Magnets
6” x 4” x 1” C8
Block Magnet
Applied Magnets
www.magnets4less.com
$5.99
And add the many other missing materials too please
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Figure 5? 6??: Magnetic field in the chamber 1cm over the Al plate (take
caption off the image below)
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References:
Alston, M., Crewe, A., & Evans, W. (1954). Some practical aspects of diffusion cloud
chamber operation. Review of Scientific Instruments , 25 (6), 549.
American Nuclear Society. (2001, June). Reactions. Retrieved January 13, 2008, from
Cloud Chamber "How-To" Tips: http://www2.ans.org/pi/teachers/reactions/200106-02.html
Bunce, G. (1996, September 15-22). The Brookhaven muon g-2 experiement.
Retrieved 12 8, 2008, from Department of Energy Office of Scientific and Technical
Information: http://www.osti.gov/bridge/purl.cover.jsp?purl=/78593S8aGzo/webviewable/
Cassidy, J. (1994). Video analysis of cloud chamber phenomena. The Physics Teacher
, 32 (February), 124-125.
Deeson, E. (2007). Collins Internet-linked Dictionary of Physics. London:
HarperCollinsPublishing.
Ketner, J. (1965). An inexpensive diffusion cloud chamber. The Physics Teacher , 3
(6), 284.
Langsdorf, A. (1939). A continuously sensitive diffusion cloud chamber. Review of
Scientific Instruments , 10 (3), 673.
Lehrman, R. L. (1968). Scientific Experiments in Physics. New York: Holt, Rinehart &
Winston.
MARIACHI Project. (2006, June 30). Cheap Gaussmeter. Retrieved December 11,
2008, from MARIACHI Project Wiki:
http://www.mariachi.stonybrook.edu/wiki/index.php/Cheap_Gaussmeter
Marquardt, N., John, V., Bein, B., & Delgadillo, I. (2000, June 26-30). European Particle
Accelerator Conference 2000. Retrieved 12 10, 2008, from CERN:
http://accelconf.web.cern.ch/AccelConf/e00/PAPERS/TUP5A13.pdf
Nave, R. (2001, July 15). Radioactive Items for Sale. Retrieved January 13, 2008, from
Hyperphysics - Georgia State University: http://hyperphysics.phyastr.gsu.edu/hbase/nuclear/nucbuy.html
Nielsen, C., Needles, T., & Weddle, O. (1951). Diffusion cloud chambers. Review of
Scientific Instruments , 22 (9), 673.
Nobel Foundation. (1965). Nobel Lectures: Physics 1922-1941. Amsterdam, The
Netherlands: Elsevier. These are online; provide a URL to the exact lecture please..
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Schuh, S., & Barradas, P. (2003, June 30). Cloud chambers in the classroom. Retrieved
April 20, 2008, from CERN:
http://teachers.web.cern.ch/teachers/archiv/HST2003/publish/
Veltman, M. (2003). Facts and Mysteries in Elementary Particle Physics. Newark, New
Jersey, United States of America: World Scientific.
Wikipedia.org. (2007, January 15). Cloud Chamber. Retrieved January 15, 2007, from
Wikipedia Web site: http://en.wikipedia.org/wiki/Wilson_cloud_chamber.html
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