How to Make a Cosmic Ray Detector
WHAT YOU NEED: scissors, black heavy felt, clean and empty glass jar with a rubber washer or cardboard filler under the lid, ruler, rubber cement, black velvet, rubbing alcohol, two towels, pie tin, rubber gloves, block of dry ice (enough to fill a pie tin), hammer, ice tongs, flashlight, three books, masking tape, small magnet.
WHAT TO DO:
1. Cut the felt in a circle to lie in the bottom of the jar. Glue the felt in place with rubber cement. Cut a strip of felt 1 inch wide and glue it around the inside wall at the bottom of the jar.
2. Cut a strip of velvet 1 inch wide and glue it to the top of the jar’s inside wall. Cut a circular piece of velvet to fit the metal lid, over the rubber or cardboard, and glue it in place.
3. Pour enough rubbing alcohol into the jar to saturate the felt at the bottom of the jar thoroughly and cover the bottom of the jar. Screw the lid on the jar tightly and let it sit for ten minutes.
4. Spread a towel on a flat surface and place the pie tin on top of it. Wearing rubber gloves (to avoid touching the dry ice with your bare hands), wrap the second towel around the block of dry ice, then break up the block with the hammer.
5. Using the ice tongs, put the cubes of dry ice into the pie tin, making a level surface on which to place the jar.
6. Turn the jar upside down and place it on the dry ice in the center of the pie tin. Position the flashlight on top of the books, attaching it in place with the masking tape, aiming the beam through the lower half of the jar.
7. Turn off the lights, and cover the bottom of the jar with the palm of your hand to warm the alcohol-soaked velvet. Watch carefully.
WHAT HAPPENS: Within five minutes, the alcohol vapor condenses, warmed by your hand, and you’ll see a continuous rain of fine mist 1 inch below the top of the chamber. After another five minutes, the rain decreases. When the interior temperature is just right, you see cobweb-like threads suddenly appearing and disappearing at various angles about an inch above the lid. These are vapor trails made by cosmic rays passing through the jar. Place the magnet against the side of the jar, and the trails will be deflected toward it.
WHY IT WORKS: The heat from your hand warms the top of the chamber, and the dry ice cools the bottom of the chamber. Somewhere between these temperature extremes, usually about 1 to 2 inches from the bottom, the air becomes saturated with alcohol vapor, and particle trails become visible where cosmic rays cause the alcohol to condense.