Illustration: Andrew J. Nilsen
Illustration: Andrew J. Nilsen

Different particle trail types you can see:

  • Thin and long tracks are muons, originating from cosmic rays
  • Thick, small tracks are alpha particles, caused by radon atoms in the atmosphere — not related to cosmic rays
  • Wiggly tracks are low-energy electrons experiencing multiple scattering
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Project Steps

Secure the felt

Inside the bottom of the cup, sandwich a section of felt between 2 magnets — one inside the cup, the other on the outside.

Prepare the cup

Wearing safety glasses, pour
in just enough isopropyl alcohol to completely soak the felt.

Form a Play-Doh seal about ½” thick on the cup rim to avoid any alcohol vapor leakage between the cup and metal plate.

Stick the cup on a very cold plate

Wearing an insulated apron and gloves, put some dry ice into the styrofoam cooler lid. Place the metal plate on top and wait until the surface becomes very cold.

Invert the cup on the metal plate and press firmly so that the cup sticks to it. That’s it!

TIP: If your Play-Doh becomes too dry after coming in contact with isopropyl alcohol, try substituting plasticine modeling clay.

Now watch cosmic rays!

With the lights off, use your flashlight to illuminate the area inside the cup closest to the plate.

After 2–3 minutes the alcohol vapor will cloud into a supersaturated state. When an electrically charged particle zips through, it will ionize the vapor atoms by ripping off the electrons along its path. This will trigger the condensation process and we can see a trail from the charged particle!