Large Electrically Cooled Diffusion Cloud Chamber with Magnetic Field

Revision:  2009-1-13

 
 
 
 
 
 

Build an Electrically Cooled Cloud Chamber for the Classroom 

Richard Gearns

Physics Department

Sachem High School East

177 Granny Road

Farmingville, New York 11738

 

Abstract

To introduce high school students to the world of particle physics, teachers can build an electrically cooled cloud chamber using off the shelf parts and easily sourced materials.  This paper describes the building of an electric cloud chamber and potential 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 and permits estimation of the momentum of observed particles.  The large size of the design allows students to explore background cosmic rays as well as radiation from a variety of source materials like radioactive sample disks, Fiestaware, and NoSalt.   

Acknowledgement

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 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

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 necessitates 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 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. 

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.  Black hard anodization provides the best combination of contrast and durability.

Thermoelectric coolers chill the chamber.  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 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 contacts the hot side. The 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.   The pump delivers 0°C water in parallel to each heat exchanger and returns the warmer exhaust water back to the ice bath 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 upon from 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.  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-4400 Kill-A-Watt meter. 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 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 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 polystyrene insulation available from Lowe’s or Home Depot with cut outs 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

Before operation of the chamber, some initial preparations must be made to insure a successful run. AOperation requires approximately 50lbs of ice, 100mL ethyl alcohol (100%), 1 meter of Mortite rope caulk, and a quality, streak-free glass cleaner should be gathered. Chamber set-up requires a dark room with space for a large camping cooler to be used for the ice bath.  A nearby sink or drain aids setup and breakdown.  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 areas that might leak water. Also, keeping potential spark away from the ethyl alcohol should be of prime concern. 

Next, clean the plastic enclosure of fingerprints and streaks using a lint free cloth and the window cleaner.  Add 100% ethyl alcohol to the 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. 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. 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 the plate and then falling toward the plate as a rain. Observation of cosmic ray events occur only when the plate temperature gets cooler than -25°C. The initial events occur at plate locations directly over the thermoelectric coolers. Although areas of the plate reaches -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. 

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. Alternatively, if this doesn't improve matters, then 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, carefully sequence shutdown of the system to avoid freeze damage. 

Classroom Activities

The increasing incorporation of modern physics concepts in introductory physics courses makes the classroom a great venue for an electrically cooled cloud chamber. Since a teacher need only acquire ice from the cafeteria and then plug-in the apparatus to initiate cooling, an inquiry into the nature of subatomic particles can occur over several days. So rather than being a "show and tell" that the teacher pulls out one day when the dry ice shipment arrives, the class can become part of a much larger investigation. 

Observing cosmic rays best utilizes the capabilities of this chamber.  However, before 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. Students 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 might hook left or right. Others may resemble corkscrews.  Still others crash their way through the cloud seemingly bumping into things and changing directions. Example images may be seen at: http://www-mariachi.physics.sunysb.edu/wiki/index.php/CCI.  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 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.  

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). 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 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)?”

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 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:

 

 
 
 
 

In addition to the cosmic ray background radiation, students can explore radiation from other 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 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, a big, impressive, and satisfying project making the invisible world of cosmic rays visible in your classroom. 

 

Appendix: 

Sourced equipment:

Qty	Brand	Model	Supplier	Cost (each)
4	Mean-Well	S-320-12	TRC Electronics www.trcelectronics.com	$70.98
4	Alphacool	NexXxos HP Pro AMD CPU Block	FrozenCPU.com www.frozencpu.com	$34.95
4	Thermal Enterprises	CP1-12726	Thermal Enterprises thermoelectrics@aol.com (732) 821-5301	$18.20
1	P3 International	P-4400 Kill-A-Watt	Amazon.com www.amazon.com	$22.94
1	Hydor-SELTZ	L35	Blue Water Corals www.bluewatercorals.com	$64.99
1	Applied Magnets	6” x 4” x 1” C8 Block Magnet	Applied Magnets www.magnets4less.com	$5.99

Figure 1: 
 
 

 

Works Cited

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/2001-06-02.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=/78593-S8aGzo/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.phy-astr.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.

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