The first cloud chamber was made in 1911 by
C.T.R.Wilson. Cloud chambers are used to show the tracks of radioactive particles rather
than to measure the intensity of the radiation.
There are two types of cloud chamber: the
expansion type and the diffusion cloud chamber. Their final results are similar but they use
different methods to achieve it.
When a radioactive particle passes through air which is supersaturated with the vapour of a liquid the ions it produces act as centres on which the liquid can condense, and so a line of liquid droplets is formed along the track of the particle. The liquid condenses more readily on the ions because they are larger than the uncharged gas molecules. The length of the track is proportional to the energy of the particle. Figure 1(a) shows an alpha source emitting alpha particles of two distinct energies. A collision with a gas atom is also visible as is the track of a cosmic ray passing across the cloud chamber. Remember that when you look at these tracks what you are seeing is a line of liquid droplets and not the ions themselves or the radioactive particles
In the expansion cloud chamber shown in Figure 1(b), the supersaturated state is produced by rapidly lowering the pressure in the chamber, the temperature drops as the air is expanded. This type of cloud chamber only shows tracks during this expansion and cooling stage.
In the diffusion cloud chamber (Figure 1(c)) solid
carbon dioxide cools the chamber so that at one level the air is supersaturated. In both cloud
chambers the liquid used is methylated spirits.
This type of cloud chamber operates
continuously as long as there is some carbon dioxide remaining. The tracks can be made
clearer by rubbing the top of the chamber with a cloth to remove excess
ions.
As was mentioned earlier the cloud chamber, and more recently the
bubble chamber, have been of considerable use in studying the tracks of particles and
hence obtaining knowledge of their relative masses.
