Building a Cloud Chamber: Why Muons Should Not Be There (page 2)

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Author: Jerry Silver

Observing tracks

  1. At first, you will notice a mist of alcohol droplets forming in the tank.
  2. After about 15 minutes, you should start to see the tracks of particles passing through the vapor a few centimeters above the base plate.
  3. It may be helpful to view the tracks by looking toward the light source at an angle so that the vapor trails are illuminated from behind.
  4. If you have a low-level radiation source (such as one of the everyday objects mentioned in the parts list), place it near the edge of the cloud chamber and compare its effects to cosmic rays. (Smoke detectors have low-level radioactive materials, such as americium, that are packaged safety for its intended use. Do not attempt to dismantle a smoke detector to get at the radioactive isotope. The mantle for at least some Coleman gas-camper lanterns contains traces of radioactive Thorium, which can also be a safe low-level source of charged particles to view.)
  5. Observe what a magnet does to the tracks. Note in particular how the particle is diverted in relation to the particle's original velocity and the north-to-south direction of the magnetic field.
  6. Once you get this going, a video camera can be very helpful in recording the tracks and providing an opportunity to analyze the tracks in detail. Still photography is very difficult because of the randomness of the way the tracks are created and the rapidity with which they fade. Extracting still images from a video recording is more likely to produce clear images of tracks.

Expected Results

After the mist forms into a supersaturated alcohol vapor, you may start to notice tracks that look like spider webs along the chamber bottom. These are cosmic rays and should be noticeable roughly several times each minute.

Alpha particles, which are two protons and two neutrons bonded together, form sharp, well-defined tracks about 1 centimeter long.

Beta particles, which consist of electrons, have thinner and longer tracks, roughly 3 to 10 centimeters in length.

Some of the tracks may come in straight and then sharply break in a different direction. An example of this is shown in Figure 125-2, which shows a muon being deflected as it dislodges an electron from an air molecule.

Building a cloud chamber. Why muons should not be here. Special relativity

A track that starts in a straight line, but then breaks off at a sharp angle, such as shown in Figure 125-3, most likely is muon decay during which a muon spontaneously decays to form an electron. The electron is visible as a thinner track. The two neutrinos do not form vapor trails and are not visible because they are not charged.


You may see a very jagged, erratic path representing a low-energy particle being scattered multiple times. This is pictured in Figure 125-4.

If you view the chamber from the front, with the particles coming from the left, and the magnet's north pole at the top of the chamber, particles bending toward the back of the chamber are positively charged particles (such as protons). Particles bending toward the front of the chamber are negatively charged.

Why It Works

Some of the radiation in cosmic rays or isotope sources consists of charged particles. As these charged particles pass through the supersaturated alcohol vapor in the chamber, the particle ionizes the molecules of the vapor. Droplets of the vapor then condense on the path left in the wake of the particle's path, leaving a visible trail.


Collisions occur that either change the motion of the particle or result in a subatomic event, which results in a whole new mix of particles. The laws of conservation of momentum and mass must be followed, which helps to identify the particles, including those that are not visible. Nonionized particles, such as neutrons, will not leave a visible track.

Figure 125-5 shows a sampling of particle collisions in a more elaborate detection system, called a bubble chamber.

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