Next obtain a short length of PVC pipe with an outer diameter that is just slightly smaller than the mouth of the canteen. Cut a hole in the cap to accommodate the pipe, which should reach down to about two centimeters above the bottom. Glue the top end into the hole in the cap. Then stretch some plastic window screening across the top of the pipe. The mesh reduces turbulence in the fluid, thereby reducing turbulence in the air inside the chamber. Secure the screening with a nylon cable tie. Then punch a small hole in the center of the screen. (You'll need this opening for access to the chamber.) Finally, glue the cap into the glass using plenty of silicone cement and attach the lower stopcock just as you did the upper one.
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WINDOWS for viewing particle tracks and for adjusting the liquid level are formed by placing tape on the glass before spraying it with black paint. |
Adding baking soda to the liquid while everything is sealed up is simpler than you might think. Just epoxy a ceramic magnet into a small bag fashioned from the toe of a nylon stocking. I suggest you begin by adding 2.5 milliliters (about half a teaspoon) of baking soda to the bag, but you'll probably need to adjust that amount after some trial and error. Place the baking soda on a small piece of paper towel and insert this makeshift holder into the bag. Affix the bag just below the neck of the bottle using a stack of two magnets on the outside; removing the outer magnets releases the baking soda into the solution. I put as many as five bags inside at once so that I can repeat the experiment without opening the canteen.
Mix two liters of liquid by combining equal parts of distilled vinegar and the most concentrated isopropyl alcohol you can find. Squirt in some ink, which (like the black spray paint) will make the tracks easier to see, and add two milliliters of salt. Fill the generator bottle with this solution to within about one centimeter of the nylon bags. Open the top stopcock and then screw the glass cloud chamber on tightly. You may need to cover the threads with Teflon tape or petroleum jelly to get a pressure-tight seal. Insert the whole assembly in its holder. Now open both stopcocks, carefully suck on the tube until liquid fills the cloud chamber about halfway, then close both stopcocks.
Position a bright light to one side of your viewing port. Drop in a bag of baking soda and monitor the fluid level in the cloud chamber, bleeding the pressure with the lower stopcock when the compression ratio rises above about 1.33. (If this ratio is below about 1.25, tracks won't materialize, and if it is above 1.38, a dense cloud forms and obscures everything.) Wait a minute or so, then rapidly open that same stopcock completely.
Particle tracks can appear only during the next brief instant, so the odds of seeing a vagabond cosmic ray are not at all good. A potent source of either alpha particles (helium nuclei) or beta particles (electrons) provides a much more satisfying show. Alpha particles produce short tracks, whereas beta particles leave long ones. You can obtain both an alpha and a beta source suitable for this project from the Society for Amateur Scientists.
Epoxy your radioactive source to the tip of a nail that has a large, flat head. Lift this assembly through the hole in the window screening and into the cloud chamber using a drinking straw and stick the head of the nail to the magnet at the top of the glass. This arrangement will hold the source securely in place until you want to swap it for another one.
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