What It Shows
The path of a single charged particle can be made visible in cooled supersaturated air/alcohol vapor.
How It Works
The cloud chamber was developed by C.T.R. Wilson at the turn of the century (he received the Nobel Prize in 1927 for his invention). When charged particles ionize a supersaturated vapor, a trail of ions is left in the path of the particles. The ions act as condensation nuclei (for the alcohol to condense on) and a thin line of fine droplets is formed in the path of each charged particle.
Our present cloud chamber is a "continuous diffusion" type1, which means that the tracks formed drift to the bottom of the chamber and are replaced continuously by new tracks. A (0.02µCi, half-life=22.3yr) source provides the ionizing radiation that produces the tracks.2 The tracks are made visible to the audience by video projection.
Setting It Up
Full instructions for the use of the cloud chamber are kept in the Prep Room files—follow them! A few additional comments: (1) the ethanol/dry-ice mixture should be quite thick...use plenty of dry ice. (2) Level the apparatus with a bulls-eye level and shims to inhibit the vapor from annoyingly drifting sideways. (3) After preparation, it requires several minutes for the air in the chamber to cool sufficiently; you may have to wait as long as 15 minutes before observing tracks. (4) Use a 25mm (or wider) lens on the video camera and shoot through the top window.
When working well, the cloud chamber will continuously produce beautiful tracks for 30 minutes to 1 hour. The range of the alpha particles (from Po-210) is about 4 cm. As the alpha traverses its path, it slows down gradually and becomes more heavily ionizing by virtue of the fact that it spends more time in the vicinity of air molecules in its path. Evidence for this can be seen by observing that the tracks become denser with increasing distance from the source. If you like, you can also demonstrate the absorption of alpha particles: Make a cylinder (approximately 1 cm in diameter) with Scotch tape and place the cylinder around the source...one or two layers of tape is probably enough to stop the alphas; if not, try some additional layers. Having stopped the alphas, it may be possible to see the (much fainter) tracks produced by the betas penetrating the Scotch tape absorber.
1 Welch Scientific Co. Cat. No. 2195; 7300 North Linder Ave., Skokie, Illinois 60077.
2 Pb-210 occurs naturally and is near the end of the uranium-238 decay chain (see Uranium Block). Pb-210 decays by β emission to Bi-210 which also decays by β emission to Po-210. Finally, Po-210 goes to the stable Pb-206 isotope (the end of the chain) by α decay (5.3MeV) and it is these alphas that one primarily sees in the cloud chamber.