Display of cloud chamber photographs

Demonstration

Displays of cloud chamber photographs are very worthwhile as a focus for discussion. As well as the straight alpha particle tracks, you can show other types to illustrate the power of the cloud chamber in the early days of studying radioactivity. Photographs showing collisions with a fork in them should provoke questions from the students as to what is happening.

You can set this up as a wall display or use digital images and a projector.

Apparatus and Materials

• Some photographs of cloud chamber tracks

Health & Safety and Technical Notes

Read our standard health & safety guidance

Even at the beginning of the twentieth century, a few stout-hearted critics maintained that the existence of atoms was not proven. It was cloud chamber pictures (in around 1911) that convinced them that there are atoms. The forks showed that there were collisions between individual particles.

Procedure

The aim is to get students to look at and discuss the cloud chamber photographs. There are a number of ways you could achieve this.

Here are some possibilities:

1. Print out sets of the photographs to give to groups of students. Ask them to look at the pictures, and arrange them according to some criteria. This could be the type of radiation, or it could be something more abstract like how easily they could explain what is happening, or how interesting the photograph looks.
2. Ask students to match captions to photographs. This could be done in groups, or you could give each photograph a number and pin them up around the room. Put the captions on a work sheet or a projector slide, and ask students to move around the display and match the photographs to the captions.
3. Make a wall display or projector slide with four photographs on it, and ask students which one they think is the odd one out. There need not be a right answer – the idea is to elicit discussion in groups or in the class.
4. Bubble chamber photographs or modern electronically produced pictures could be added to the display.
5. You could use a PowerPoint file rather than a wall display.

Teaching Notes

• Although students may see these cloud chamber photographs before they meet the Rutherford scattering experiment, Rutherford’s experiment came first. He did not have the evidence from these photographs when he developed his model of a hollow atom with a tiny, massive nucleus.
• The paths of nearly all the alpha particles do not bend, despite their many collisions. Therefore, they must be much more massive than the electrons which they pull off. In these collisions, the alpha particle passes through the hollow part of the atom and is nowhere near what is now called the nucleus.
• Very rarely, an alpha particle collides with something more massive (Rutherford’s nucleus) producing a forked track. The rarity of these events is an indication that the nucleus is much smaller than the atom.
• In collisions with a hydrogen nucleus, the alpha particle and the hydrogen nucleus both continue forwards. This indicates that the alpha particle has more mass than the hydrogen nucleus. In collisions with nitrogen and other heavy atoms, the alpha particle sometimes bounces backwards, showing a collision with something more massive. In collisions with helium nuclei, the fork shows a 90° angle, which indicates an elastic collision with an object of equal mass.
• Bubble chamber photographs or modern electronically produced photographs can be added to the display.

This experiment was safety-tested in May 2007