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The Physics of Radiation: Vapor Trails in the Mist



What is radiation? Is all radiation harmful? Can we see radiation? Radiation is everywhere; and is an essential part of nature. Radiation can be broken down into 2 categories: Ionizing and non-ionizing. Non-ionizing radiation is low energy. Common  examples are: visible light, microwaves, and radio waves. Ionizing radiation has higher energy than non-ionizing and is capable of stripping electrons from atoms creating ions. Examples include: alpha and gamma rays, x-rays, and cosmic rays. What about the harmful effects of radiation? It’s true that radiation (in large quantities) can be harmful, but it is also true that living organisms need radiation to survive. With the help of an instrument called a cloud chamber we can visualize radiation. Let’s build one and see if we can identify a radioactive particle.

Note: Students ask your parents for help with this experiment. Dry ice is very cold and can cause burns if touched with bare hands


The supplies needed for this experiment are:

  • Clear plastic cup
  • 1 lb of dry ice (use gloves or tongs; do not touch with bare hands!!)
  • Felt
  • Flashlight
  • Clear plastic wrap
  • Rubber band
  • High purity rubbing alcohol.

Line the bottom of the cup with the felt. Place a liberal amount of alcohol on the felt, then place a piece of clear plastic wrap around the top of the cup to seal off any outside air. Use the rubber band to secure the plastic wrap. Now, hold the sealed cup between your hands for about 30 seconds to warm it up. This will help the alcohol vapors saturate the air inside the cup. Turn the cup upside down and place on the dry ice. After a few minutes, turn off the lights and close the curtains, to limit the available light. Using a flashlight, shine a light beam through the side of the cup. Focus the light on the bottom 1/3rd of the cup (closest to the ice). We will start to see a “mist” of alcohol vapor form. This mist forms as the alcohol supersaturates the air. Approximately 3 minutes after the mist forms we will start to see vapor trails form in the mist.


As the air becomes saturated with alcohol vapors, muons interact with the vapors. Muons like electrons are negatively charged but have a much higher mass (approx. 200 times more massive than an electron). This interaction strips the electrons from (ionizes) the atoms. This separation of the charged particles will attract alcohol vapors causing them to condense. What we see are the vapor trails left by the muons as they pass through the saturated alcohol vapor. We saw some very thin lines, and we saw some heavier thick lines. We also might have seen some lines with kinks in them.

Muons are formed as a by-product of cosmic rays (high energy protons) which interact with molecules in the earth’s atmosphere. The resultant muon particles are what we see in the cloud chamber.

Fun Fact:

An experiment with muons, in 1941, lead to the first scientific proof of time dilation which was predicted by Einstein’s Theory of Special Relativity.


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