4.Main Content
experiments
Expansion cloud chamber
Demonstration
This is a useful demonstration to introduce students to what they can expect to see in the diffusion cloud chamber, and possibly to see forked paths resulting from collisions.
Apparatus and materials
• Expansion cloud chamber
• Source of alpha radiation (if it is not part of the cloud chamber)
• EHT power supply (NOT an HT one)
• Large forceps or pliers, if required
• Bicycle pump or other device for producing expansion
• Illuminant (lamp, lens and power supply)
Technical notes
Various types of expansion cloud-chamber are available commercially. They differ greatly in effectiveness and clarity for small groups of students so it's advisable to 'try before buying'. Much depends on the illumination. Some require alcohol, but those which work with water are preferable.
i)
ii)
The above diagrams show a Princeton University design used (i) for a few students viewing directly; or (ii) for projecting a large image by an overhead projector.
The above diagram shows another design.
All types need an electric field to sweep away ions left by earlier events. In each case the manufacturer's instructions should be followed carefully.
Expansion cloud chambers are relatively expensive so schools are unlikely to have more than one.
By contrast, the Taylor diffusion cloud chamber is inexpensive and can be used for class experiments. Also, it maintains a constant state of supersaturation (see Diffusion cloud chamber).
Safety
See guidance note on Radioactive sources (UK guidelines*).
NB The radioactive sources supplied with some expansion cloud chambers screw into the base of the chamber and are moderately strong. Handle them with large forceps or pliers.
Read our standard health & safety guidance
Procedure
Expansion cloud chambers are all slightly different in their operation, and you will need to refer to the manufacturer's instructions. The position of the lamp is often critical.
Some have an evacuation mechanism, such as a bicycle pump, which removes the air whilst others change the pressure of a water column.
Teaching notes
1 The tracks produced in cloud chambers are always fascinating and a forked track even more exciting. If the cloud chamber is filled with helium gas then alpha particles will produce a 90° forked track. Alpha particles in a chamber filled with air will be deflected at more than 90° (oxygen and nitrogen have almost the same atomic mass, about four times that of alpha particles).
2 Display collections of cloud-chamber photographs, keeping them on view for some time. Include examples of fork-tracks resulting from collisions and in particular an example of a 90° fork of an alpha-particle collision with a helium nucleus.
3 The first cloud chamber was invented in 1895 at the Cavendish Laboratory by the Scottish physicist CTR Wilson. He was trying to imitate the way that clouds formed as wet air cooled on expansion. Initially, he used it to examine clouds and condensation rather than to track ionising radiation. He invented it to save him having to climb up to the observatory on the top of Ben Nevis. It wasn’t until 1911 that he developed the cloud chamber as a measuring device and photographed the paths of alpha and beta particles. For this he was awarded the Nobel Prize in 1927.
The Wilson cloud chamber works by expanding a fixed volume of wet air. The air cools as it expands, forming a supersaturated vapour. The vapour will condense into droplets if it is provided with condensation nuclei, such as ionised air molecules. In this way, the cloud chamber produces a visible trail of droplets left behind by, for example, an ionising alpha particle.
The air will soon warm up again. So the results are fleeting. For this reason, the Wilson cloud chamber is sometimes known as the pulsed cloud chamber.
Cloud chambers and even their successors, bubble chambers, are now no more than historic curiosities. Analysis of cloud chamber photographs assumes that momentum is conserved in nuclear events. A uniform magnetic field applied to a cloud chamber perpendicular to the direction of motion of the charged particles produces tracks which are bent into a circle. From this, the momentum of the particle can be calculated and hence its velocity if its mass is known.
The kinetic energy before and after collision can then be calculated and if it is not conserved either it was a rare inelastic collision or the target nucleus was wrongly identified (due to impurities or the liquid used to make the drops). If it was an inelastic collision then a nuclear transformation has been effected by the alpha particle.
This experiment was safety-tested in April 2006