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. It 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 incrase 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 and so if it drops the temperature drops and this leads to supersaturation. Then when an ionizing particle moves through the chamber the ionized particles it leaves behind provide condensation nuclei for the vapour to for a trail of condensation around. After this the chamber is returned to its original volume. These tracks can then be photographed for further observation to determine the nature of the particle that caused to trail and, if a magnetic field was applied, its charge. Later on a different chamber, the DiffusionChamber? , was developed that used a cold source to cool the chamber from the bottom giving a temperature gradient so that reseting the chamber is no longer required to be reset after a short period of time but can continue to operate as long as the cold source exists.

Development of the Cloud Chamber

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

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 adiabtically 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.