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More cloud chambers

These plans were originally published in Science Experimenter by the Arco Publishing Company in 1964. Due to a high volume of requests, I am including them here. Please keep in mind that these are 35-year-old plans and some parts may need to be substituted. I have not personally built either of these chambers, so you're on your own!

CLOUD CHAMBERS
for tracking nuclear ghosts
Complete plans for building and operating two
prize-winning, science fair cloud chamber projects

Part 1: Expansion type chamber

EVERY science fair, whether it be local, state or nation wide usually has one or more cloud chamber projects built and entered by students interested in nuclear physics. And, in some cases these students have taken top awards in their particular fairs.
Basically a cloud chamber is a sealed transparent chamber containing a supersaturated atmosphere of air and water or alcohol vapor which, when conditions are just right, make it possible to see and photograph and thus study the vapor trail of an atomic particle. Usually the source of these particles-a minute piece of radioactive material-is placed within the chamber. However, some atomic particles - cosmic rays for example can penetrate the walls of the chamber. When you see the track of a stray cosmic particle you are witnessing the concluding phase of an event which may have had its origin millions of years ago in an exploding star!

The First Cloud Chamber.

In 1927 a British physicist, Dr. C. T. R. Wilson was awarded the Nobel prize for the invention of the cloud chamber itself in 1911 and some of the discoveries he later made with it.
There are two basic types of cloud chambers in use today; (1) the expansion chamber (Fig. 6A) which depends upon expansion of the air, within the chamber, to cool it and thus create a supersaturated atmosphere, and (2) the continuous or diffusion type chamber (Fig. 6B), that creates a supersaturated atmosphere bydiffusion of vapor from the warm upper region of the chamber to the cooled bottom of the chamber. In both types, electrically charged ions created by nuclear particles colliding with atoms within the chamber atmosphere, serve as nuclei of condensation.

The vaporized water and/or alcohol, which makes up the supersaturated atmosphere in the chamber, then condenses in the form of tiny but visible droplets on these charged ions. Since the nuclear particle leaves a path of charged ions, it is this string of droplets that you see as the vapor trail or track. All dust and other ions, which could also serve as nuclei of condensation, must of course be removed from the chamber beforehand. The charged ions are swept from the chamber by a battery operated electric-field setup within the chamber (Fig. 6).

Materials

  • 1 4 1/2" I.D. x 5" O.D. x 6" long Lucite tube
  • 1 1/4" x 8" x 8" Plexiglas
  • 3 1/4" x 6 1/2" x 6 1/2" Plexiglas
  • 1/2 pt Plexiglas and Lucite solvent
  • 3 3/4" x 5 5/8" x 24" pine or hemlock
  • 1 3/4" Pipe floor flange
  • 1 3/4" Pipe close nipple
  • 1 3/4" pipe tee
  • 1 3/4" x 1/2" pipe bushing
  • 1 1/2" X 3/8" pipe bushing
  • 1 3/4" x 1/4" pipe bushing
  • 1 3/4" self-closing, oil drum faucet
  • 1 metal auto tire valve stem
  • 1 5" x 10" copper window screening
  • 1 6" x 6" black velveteen
  • 1 plastic household sponge
  • 1 12" x 12" rubber dam
  • 2 #0 rubber stoppers with one hole
  • 5 ft plastic insulated, stranded wire
  • 24 6-32 x 1" rh. machine screws with nuts and washers
  • 4 " x 1" rh. machine screws with nuts and washers
  • 1 4-40 rh. machine screw with nuts and washers
  • 1 1/16" x 12" x 12" sheet cork
  • 2 240-volt photo-flash batteries
  • 1 tube of Pliobond cement

Building an Expansion-type Cloud Chamber.

After obtaining the materials listed in Fig. 3, lay out parts A, B, and D. If you wish to have the top of the chamber removable, substitute parts E and F for part D. This alternate top construction will enable you to quickly remove the top for cleaning without disturbing the velveteen background piece. You will notice that both sides of the Plexiglas pieces are covered with paper when you receive them. This is to prevent scratching the material while working it. Make pencil layouts directly on the paper and drill and saw it before removing the paper. Use a jigsaw or hand coping saw to cut the Plexiglas and when drilling, feed the drill very slowly to avoid cracking the Plexiglas. Lay out and drill the 1/4 and 1/2 in holes in the tube as in Fig. 3 .
To fasten the tube to the base flange piece B and top D or E, pour a small amount of Plexiglas solvent on a clean piece of glass. Set one end of the tube in the solvent and move it around on the glass slowly, lifting it occasionally to make sure the entire edge is moistened and thereby softened by the solvent. While the tube is on the glass, remove the paper from one side of part B and, with an artist's paint brush, apply solvent on the part that will he in contact with the tube. Try not to get any of the solvent on areas not covered by the end of the tube because the solvent will mar the transparency of the plastic. Make sure, too, that your fingers are not moist with solvent when handling the plastic. The solvent evaporates quickly so you may have to add some to the glass.
When the plastic has softened, which takes about two minutes, quickly set the tube on flange B and place a board and weight (about 10 lbs.) on top of the tube. Let stand for several hours to dry and then repeat the procedure cementing the top piece D or E, if removable top is wanted, to the tube.
Cut three triangular pieces of Plexiglas (G in Fig. 3 ) and cement to the inside of the tube to support the velveteen, screen and sponge assembly.
While the cement is drying, make up the in. pipe fitting assembly (H in Fig. 3 ), using pipe compound on all male pipe fittings. Then cut off the large end of a metal auto tire valve with a hacksaw, remove the valve stem and solder to the 1/2 x 3/8 in. pipe bushing. Use a propane torch to flow the solder around the tire valve. Replace stem after valve cools.
To support the chamber, make the wooden stand (Fig. 3 ) and fasten plastic piece A to it Mark and cut a sheet-cork gasket to fit the floor flange of the pipe assembly and fasten the pipe assembly to part A, using gasket compound. Use washers on the in. bolts and tighten each one a little at a time to avoid uneven pressure on the gasket.
Now, getting back to the tube, cut a 1/2 in. wide strip of heavy-duty, aluminum foil long enough to go around the plastic tube. Fasten it to the top, inside of the tube, with Pliobond cement and insert the 4-40 machine screw in the in. hole using a folded wad of aluminum foil under the screw washers on each side of the tube. Since the screw is the terminal post for the foil strip it must make contact with the foil on the inside of the tube and still be airtight. Use Pliobond cement around the screw to seal it.
To support the velveteen background and serve as the plus terminal of the electric sweep field, cut two discs from copper fly screening that will just fit inside the plastic tube. Solder the two pieces of screening together with a few dabs of solder around the edges. Also solder a 2 ft. length of plastic covered, stranded wire to the screening. Then cover the screening with a circular piece of black velveteen, wrapping it around the edges of the screen about in. Fasten by stitching through the screening with black thread. Next cut a ring from a plastic dish cloth as in Fig. 3 , and stitch it to the underside of the screen around the edges.
Now, using the bottom flange on the tube as a pattern, mark and cut a ring-shaped sheet cork gasket A piece of in. metal tubing with one end filed to a sharp edge will serve as a punch to cut the bolt holes in the gasket Also cut a disc (not a ring) from the rubber dam material to cover the entire bottom of the chamber. This rubber disc (J in Fig. 3 ) serves as a diaphragm to vary the pressure inside the chamber. If you have made the removable top, cut a cork gasket for it as you did for the bottom.
To assemble the cloud chamber, place the rubber dam over part A, lining up the bolt holes. Then place the cork gasket over the dam. Next, insert the velveteen-screen assembly into the top of the tube, resting it on the three triangular pieces of plastic. Force the sponge tightly against the walls of the tube with your fingers and, if you intend to operate the chamber immediately after assembling it, saturate the sponge with about 20 drops of methyl alcohol.
Skin the insulation from the stranded wire soldered to the screen where the wire will pass under the lower flange on the tube as in Fig. 3 . Untwist the strands so they can be arranged side-by-side. A dab of Pliobond cement at the point where the wire passes between the gasket and flange will make an airtight seal. Do not use gasket compound on the lower or upper gasket. Use washers on all tube flange bolts and tighten each a little at a time until all are drawn up uniformly.
To measure the pressure inside the chamber a manometer is used. You can build one yourself as in Fig. 4. Any transparent, flexible plastic tubing of at least 1/8 in. inside diameter may be used. The plastic tubing used for intravenous feeding will do. You can pick this up at your local hospital supply house. Leave the rubber bottle adapter on one- end of the tubing for use as a funnel, and short length of rigid tubing on the other end to insert into the hole in a #0 rubber stopper. Place the stopper in the lower in. hole drilled in the chamber. Then pour enough mercury into the manometer tubing to fill the U-shaped bottom up to about the 10 in. mark.
Before using the chamber, test it for air leaks by connecting a car tire or football pump to the tire valve soldered on the piping, and pump up the diaphragm about 1 in. Then, coat all piping, gasket and cemented seams with soapy water. Any air leaks will show up as soap bubbles. If air leaks where the plastic has been cemented together, apply solvent to the seam with an artist's paint brush until the plastic softens.

Operating the Expansion-type Cloud Chamber.

You will need a very small amount of radioactive material to place in the chamber. Experiment with material that emits alpha rays first, because they are the strongest ionizing agents and their vapor trails are easiest to see. You can purchase a radio-active needle from a scientific supply house, or chip off a small piece (about 1/16 x 1/16 in.) of luminous paint from an alarm clock dial and cement it to the end of a length of 1/8 in. wooden dowel inserted through the hole in a #0 rubber stopper. Be sure the cement does not cover the top of the paint chip. Place the stopper in the top in. hole in the chamber so that the end of the dowel projects about 1 in. into the chamber.
The electric sweep field is powered by two 240 volt photo-flash batteries. Connect the batteries to the aluminum strip at the top of the chamber and the copper screen as in Fig. 6A. Handle the battery wires one at a time and carefully to avoid a shock. Although the amperage is very low the voltage is high, and you can get a surprising but harmless jolt.
Illuminate the chamber with a 300-500 watt movie or slide projector, to provide a brilliant yet cool light. Place the projector about 1-1 ft. from the chamber so that the beam of light enters the chamber through the side of the tube above the velveteen background.
Now, pump up the diaphragm until the mercury in the manometer is lowered about 3 in. Then quickly release the air by suddenly opening the oil-drum valve. The moment the air was released you should have seen fog form in the top of the chamber. If a fog did not form, increase the pressure about in on the manometer and again release quickly. Repeat the process slowly increasing the pressure until fog is formed in the upper chamber. Allow about a minute between each expansion to give the liquid (methyl alcohol or water) enough time to evaporate into the chamber atmosphere.
If more alcohol or water must be added to the chamber, squirt some on the sponge and velveteen using an eye dropper inserted through one of the stoppered holes.
Fog is formed because the air within the chamber is cooled when the pressure is quickly dropped. Since warm air will hold alcohol or water vapor to the point of saturation, it becomes supersaturated when cooled and the excess vapor tends to condense out of the air. At first the excess vapor condenses on dust particles which are present in the air. These fall to the bottom of the chamber, due to the pull of gravity, and carry the dust particles with them. This accounts for the fog you see during the first 10-20 expansions. The vapor will also condense on ions (charged particles) in the air. For this reason the battery-powered electric sweep field is set in the chamber to attract these scattered ions and remove them from the chamber.
Heavy vapor tracks of alpha particles radiating from radium source placed within expansion-type chamber. Note that some of the tracks are curved due to turbulence of atmosphere within chamber.
With the dust and scattered ions removed from the chamber interior, the only nuclei left for the supersaturated air to condense upon will be the ions created by the alpha particles and any stray atomic particles that may penetrate the chamber walls. It is the condensation on these ions that you will see in the form of tracks as in Fig. 5. Don't expect to see tracks after the first few expansions. If alcohol is used, time the expansions about one minute apart; if water is used, space the expansions about two minutes.
Other radioactive sources emitting beta or gamma particles can be substituted for the alpha source and their tracks studied. You can set up barriers of various materials in the chamber to experiment with the penetrating power of the different atomic particles. If you place a freshly polished needle of zinc in the chamber, you should see beta tracks shooting from the needle point. These are caused by photoelectrons released by light shining on the zinc metal. Other metals can also be tested by using an arc lamp to increase the ultraviolet light. Setting up various experiments like those mentioned and observing the results is the type of research you can conduct with a cloud chamber. Taking notes and making a record of your experiments may win you a science fair award.

Original cloud chamber page Continuous type cloud chamber
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