The Cloud Chamber

A Cloud Chamber is a device used to detect ionizing particles and to determine their trajectories. It does not show the particles themselves, but where they have been. A cloud chamber is in effect a sealed chamber that is cooled so that supersaturation of alcohol or water vapour will occur within it. This means that when an ionizing particle travels condensation will occur about the nucleus of the ions it produces- these are called condensation nuclei, as the vapour in the chamber is on the verge of condensing, much in the same way that water vapour condenses around dust particles in the atmosphere to form clouds. This condensation trail leaves a fine mist that we can see which tells us where the particle was/originated from and the path it has taken from then. Some pictures of the Birmingham fish tank cloud chamber are shown here.

How does it work?

The first cloud chamber used air saturated with water in a glass chamber, the bottom of this chamber could be pulled down to increase the volume of the chamber causing the gas and vapour within it to expand as well, and as such do work, however this change is adiabatic- involves no heat transfer. So from the first law of thermodynamics we know that the energy for this expansion has to come from somewhere else, in this case the internal energy of the gas. The internal energy is related to the temperature of the molecules in the gas, so if the chamber expands the temperature drops causing the water vapour to become supersaturated. Then when an ionizing particle moves through the chamber the ionized particles, which it leaves behind, provide condensation nuclei for the vapour to form a trail of condensation around. After this the chamber is returned to its original volume.

The tracks can then be photographed for further observation to determine the nature of the particle that caused the trail, frequent changes of direction suggest frequent interactions with gas molecules, behaviour normally shown by alpha particles. An electric or magnetic field can be applied across the chamber which will cause charged particles to curve, positive and negative particles curve in different directions, positive particles turn to the left whereas negative particles turn right, making them distinguishable from each other.

C. T. R. Wilson, Proc. Roy. Soc. A, Vol. 87,595, pp.277,

http://www.jstor.org/stable/93225

Development of the Cloud Chamber

Wilson was interested in the formation of clouds, and in electrical and optical phenomena associated with clouds. He developed the chamber in 1894-5, as a way of making small clouds in the laboratory. His cloud chamber contained a saturated vapour, ready to condense into liquid droplets. He used it for a variety of investigations into the formation and disappearance of liquid droplets from saturated vapour.

In the first decade of the twentieth century, his fellow physicists were working on the task of identifying and describing the sub-atomic particles emitted by radioactive materials. Wilson realised in 1910 that his cloud chamber could contribute to this study. If an ionising particle passed through saturated vapour, the vapour would condense into droplets along the path of the particle, where ions provided nuclei for the condensation process.

Wilson’s original design was modified by Patrick Blackett, who included a spring-mounted diaphragm that could be moved up and down several times per second. Each compression-decompression cycle provided the right conditions for particle tracks to form. Given the time and patience required to identify new and interesting tracks, this was a useful way of speeding up research work.

Then in 1936 the American nuclear physicist Alexander Langsdorf came up with a variation on the cloud chamber, the diffusion chamber. It uses a cold source, usually dry ice, to cool the chamber from the bottom giving a temperature gradient, meaning that there is a region which is always supersaturated allowing particles to be detected continuously, rather than only immediately after a pressure reduction.

The Science Behind it All

This is in effect all down to conservation of energy and is often formalised as ΔQ= ΔU + ΔW

where Δ is the Greek letter delta symbolising a change of.

Q is the heat transferred into or out of the system. Heat transferred into is positive heat transferred out is negative

U is the internal energy of system.

W is work done on or by the system, work done by is positive work done on is negative.

We then know that work= force multiplied by the perpendicular distance W=F x d

In a gas P=F/A which can be re written as PA=F

P being the pressure

so pressure multiplied by area= the Force Where A is the cross sectional area of a cylinder etc.

So we now have:

W=PA x d

however we also know that if we multiply an area by a distance we get a volume so in one final step we can say that:

W=PV

So if we get an increase in the volume then work is done by the gas. We can then also find that from the kinetic theory of gases that the total kinetic energy of an ideal gas is related to the absolute temperature of the gas, so the internal energy of the gas is related to the absolute temperature of the gas.

KE=3/2NkT where k is the Boltzmann constant and N the number of molecules.

So now our original first law of thermodynamics can be re-written as:

ΔQ = Δ3/2NkT +ΔPV

So since the chamber expands adiabatically there is no heat transfer, so ΔQ=0. However the volume has increased and so the system has done work, ΔW is positive, this means that to conserve energy the internal energy must be negative and of the same magnitude as such there must be a temperature drop- since U is related to T as shown above- and it is this that causes supersaturation within the chamber.