Concepts

Particle detectors, cosmic rays, radioactivity, condensation

Overview

The cloud chamber detects secondary cosmic rays and radioactive decays by making alcohol clouds along the particle paths through the chamber. Particles ionize alcohol molecules as they pass through saturated alcohol gas, making the polar alcohol molecules condense and form clouds around the ions.

Details

Equipment

• [1] Small cloud chamber
  
• [1] LED desk lamp
• [1] Thorium source
  
• [1] Bottle of ethanol
  
• [1] Box of dry ice
• [1] Hammer
  
• [1] Spoon

Safety Equipment

• [1] Safety glasses
• [2] Safety glove

Classroom Assembly

1. Remove the plastic wrap top and soak the chamber's sponges in ethanol. Spray a little ethanol on the bottom plate too.
2. Place the thorium source in the chamber.
3. Close the plastic wrap top, pressing on the tops of the walls to ensure an adequate seal.
4. Lift the main part of the chamber and uncover the base.
5. Put on the gloves and safety glasses.
6. Place an even layer of dry ice on top of the base using the gloves and/or the spoon. Hammer the dry ice in the box to loosen it up, if required.
7. Place the main part of the chamber onto the dry ice.
8. Place the cloud chamber and lamp so the chamber can be illuminated. The lamp should be at an angle to the vertical.
9. Turn on the lamp. The chamber will take a few minutes to cool down.

Important Notes

• Dry ice can cause severe burns and blindness. It can also cause deadly CO₂ poisoning in confined or poorly ventilated spaces. Use with caution.
• The dry ice layer needs to be fairly even for the chamber to work.
• Use caution when using the thorium source. Wash hands after handling.

Script

View from behind the lamp, if possible, to avoid glare.

There is not really much to do once the demo is set up. You just need to talk about the physics of the cloud chamber and what it detects. There is a lot you can talk about, though.

• Particle physics and relativity
  You can try to identify the specific particles from the shapes of the tracks, though it can be very difficult. Electrons, positrons, and muons tend to make thin tracks,  though this may be hard to see sometimes because the thin tracks can thicken quickly as they form. Electrons and positrons should have a greater tendency to bounce around since they are low-mass, but this isn't always seen. The muons actually have to be travelling at relativistic speeds to reach the cloud chamber because without time dilation, they don't last long enough.
  Alpha particles are high-mass and make straight, thick tracks. Alphas come from radon gas emitted by radioactive minerals in the ground, largely originally uranium. The thorium source is from an old lantern mantle used to increase the brightness of camping lanterns. The thorium-232 in there is an alpha emitter, but there are alpha and beta emitters down the decay chain.
  
• Cosmic rays and radiation
  High-energy particles (often protons) from the sun and distant astromical objects (e.g. stars, black holes) are primary cosmic rays. When they hit Earth's atmosphere, they create a shower of particles called secondary cosmic rays. These secondary particles (such as electrons, positrons, and muons) can be detected by the cloud chamber. The cosmic ray electrons tend to follow Earth's magnetic field to the poles, where they excite nitrogen and oxygen in the air to make the auroras.
• Particle detector history
  The cloud chamber and its descendents are integral to at least 6 Nobel Prizes in physics:
  1927: Charles Thomson Rees Wilson for the invention of the cloud chamber
  1936: Carl David Anderson for discovering the positron using the cloud chamber
  1948: Patrick Maynard Stuart Blackett for improvements to the cloud chamber method and discoveries in nuclear physics and cosmic radiation
  1960: Donald Arthur Glaser for the invention of the bubble chamber
  1968: Luis Walter Alvarez for development of the hydrogen bubble chamber and subsequent discovery of many resonance states (short-lived particles)
  1992: Georges Charpak for development of particle detectors, particularly the multiwire proportional chamber
  

Additional Resources