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More cloud chambers, continued

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 2: continuous or diffusion type cloud chamber

Matrials List

  • 1 1/8" x 8" x 8" aluminum sheet
  • 1 6 " x 6 " window glass
  • 1 plastic sponge
  • 1 3/4" x 48" strip of 22 gage copper
  • 1 thumbtack
  • 5 ft plastic covered, stranded wire

In 1939 Dr. Alexander Langsdorf, Jr., then of the University of California at Berkeley, and later with Argonne National Laboratory, Chicago, developed a new type cloud chamber capable of maintaining a supersaturated atmosphere continuously for hours instead of for only a split second as the expansion-type chamber mentioned previously.

Plans for the continuous cloud chamber shown in Figs. 7 and 8 are the result of building and experimenting with 8 different chambers during the past 2-3 years by Eric D. Schneider.

The supersaturated atmosphere in this type of cloud chamber is maintained continuously by the evaporation of methyl alcohol at room temperature placed at or near the top of the chamber. The bottom of the chamber is a metal plate cooled by dry ice, which is about -70° C. As the alcohol in the saturated sponge (Fig. 6B) evaporates and diffuses downward, toward the cold surface, the atmosphere within the chamber becomes supersaturated somewhere between the top and bottom, and will remain supersaturated all the way to the bottom. In this way a layer of atmosphere is created that is ideally suited for the sighting of atomic-particle vapor trails.

Building a Continuous-Type Cloud Chamber

This type of chamber is far simpler to build and will cost less for material than the expansion type chamber. However, since dry ice (which costs about $1 for a piece large enough to operate the chamber) must be purchased each time the chamber is used, the total cost of conducting experiments can run to more than using the expansion type chamber.

Start construction by cutting off the top and bottom of a clear-glass, 1 gal. jug. If a wide mouth, 1 gal. pickle jar (obtainable from restaurants) is used, only the bottom need be cut off. Make a cutting jig from scrap wood as shown in Fig. 11 and, holding the jug firmly against the backstop, slowly rotate the jug against the clamped glass cutter. This will score a line completely around the jug. The height of the cut-off portion of the jug should be about 5 in. Now, stand the jug upright and heat the scored line with a propane torch while slowly turning the jug (Fig. 12). After heating the jug for about 1 minute, lightly tap it along the scored line with the tip or nozzle of the torch. It should crack off clean at the scored line. To smooth the cut edge of the jug, tack a sheet of 80-100 grit silicon carbide abrasive paper to a flat board and, with the cut edge of the jug on the paper (Fig. 13) slowly move the jug around in a circular motion without rotating it.

Make the base of the chamber from an 8 x 8 in. sheet of aluminum about 1/8 in. thick. Saw off the corners and drill the three holes as detailed in Fig. 8. Apply 3 coats of flat black paint to what will be the top of the plate, and fasten an ordinary thumbtack, so that the point projects through the center 3/8 in. hole, with household cement and cellophane tape. The top is simply a piece of double-strength window glass cut octagon in shape as in Fig. 8.

To absorb and hold the alcohol, fasten a 1/4 x 5/8 in. strip or strips of plastic sponge with Pliobond cement around the inside of the jug, 1 in. from the top edge. The gasket around the top edge can be a U-shaped rubber gasket salvaged from a washing-machine gasket.

For the upper (cathode or minus) side of the electric sweep field, solder the ends of a in. wide, 22 ga. strip of copper so that it will fit snugly inside the jug or chamber. Solder a 3 ft. length of plastic-insulated stranded wire to this strip of copper and fasten the copper strip inside the jug with Pliobond cement, 1 in. from the bottom edge of the jug.

To assist in absorbing and transferring heat outside the jug to the alcohol-saturated sponge inside the jug, make up a copper band similar to the one on the inside and fasten it to the outside over the sponge as in Fig. 8.

Operating the Continuous-Type Cloud Chamber

For a radioactive source, emitting alpha particles, scratch the point of the thumbtack fastened to the base across the hand of a luminous-dial clock, picking up some of the material on the tack. Then coat the bottom edge of the jug with petroleum jelly (use quite a bit), thread the wire from the inner copper band through the 3/32 in. hole in the base and set the jug on the base. The petroleum jelly will seal the bottom edge froth outside air currents which would obstruct and even destroy fine vapor trails.

Now, place the base, together with the jug, on a 10 x 10 in. block of dry ice 1 or 2 in. thick (Fig. 7). The insulated paper wrapping around the dry ice can be left intact and a hole large enough to take the base cut in the top side. (Caution, use gloves when handling dry ice because it is cold enough to "burn" your fingers.)

Immediately after setting the base on the ice, swab the inside of the base with cotton saturated with alcohol. This will increase the intensity of the black background and make viewing better. Then, using an eye dropper, saturate the sponge inside the jug with 90% pure methyl or ethyl alcohol, and set the glass cover in place on top the jug. Hook up plus terminal of a 240V. photo-flash battery to the machine screw on the base and the minus side to the wire leading to the copper band inside the jug. Set up a 300-500 slide projector so that the light beam enters the side of the chamber just above the base.

After about 15-20 minutes enough of the alcohol should have evaporated to cause super-saturation of the atmosphere in the lower half of the jug or chamber. If you keep changing your angle of vision with respect to the light beam, you should at one point see what appears to be a fine mist or rain falling to the bottom of the chamber. It is in this zone that you will also see the vapor trails of atomic particles. If you do not get results within an hour, heat the outside copper band and upper atmosphere in the chamber for a minute or two with an electric light bulb in a reflector or a reflector-type, photoflood bulb.

Most of the tracks coming from the tack scratched on the luminous clock dial will be made by alpha particles (Figs. 9 and 10), which are high-speed helium nuclei consisting of two protons and two neutrons. Their velocity is about 1/20-1/25 that of the speed of light (speed of light is 186,000 miles per sec).

Alpha vapor trails are thick, well-defined straight tracks, seldom bending. Their lengths will be from 1-3 in. and the vapor trails will slowly sink to the base and disappear immediately after forming.

Since a certain amount of detail is regrettably but unavoidably lost in the printing reproduction process, however, the tracks will be well defined and much clearer than those shown in Figs. 9, 10 and 14. Note: these images not included here due to lack of clarity. -ed.

Beta particles are high-speed electrons having a velocity nearly .99 that of light. Their trails in a cloud chamber appear as very thin irregular tracks because far fewer ions are formed per unit of length as compared with trails left by alpha particles.

Often tracks can be seen that are obviously not caused by radiation from the thumbtack. These trails are from secondary cosmic radiation, as at left in Fig. 9. The chamber can also be operated without placing a radioactive source within it. Any tracks then observed will be of primary or secondary cosmic radiation (as shown in Fig. 10) that have penetrated the walls of the cloud chamber.


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