-- NigelWatson - 22 Sep 2010

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, as particles form a condensation trail in the chamber which leaves a fine mist that we can see which tells us a particle's path through the chamber. Some pictures of the Birmingham fish tank cloud chamber are shown here.

[unnumbered] page of Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, Vol. 87, No. 595, Sep. 19, 1912Original cloud chamber tracks

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

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 within it to expand as well, and as such do work. However this change is adiabatic- involves no heat transfer. The first law of thermodynamics states energy is conserved and cannot be created or destroyed, so we know that the energy for this expansion has to have come from somewhere; 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. This brings the water vapour close to condensing, making it become supersaturated (see Appendix for more details). If an ionising particle, such as alpha or beta radiation, passes through this vapour then the ions formed acts as points of condensation for the surrounding vapour, leading to the formation of visible clouds.

More modern cloud chambers work differently to the original apparatus, as they use alcohol instead of water and do not change the volume of the chamber but instead use dry ice to cool the base of the chamber. The alcohol is soaked in a tissue at the top of the chamber, which is much warmer than the chamber base; the alcohol vapours therefore fall to the base of the tank, where they reach a point of supersaturation. Ionising particles that pass through the vapour shows up in exactly the same way as with the original water detailed above.

The cloud tracks can be photographed for further observation to determine the nature of the particle that caused the trail; for example, frequent changes of direction suggest frequent interactions with gas molecules, which is normally shown by alpha particles (the most ionising form of radiation). 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, making them distinguishable from each other.

Development of the Cloud Chamber

Year DevelopmentSorted ascending
1936 Alexander Langsdorf modified the chamber to produce its modern variant, the diffusion chamber. Using dry ice to form a temperature gradient meant there was always a supersaturated region, so particles could be detected constantly
1932 Blackett and Giuseppe Occhialini developed a system of Geiger counters which only took photgraphs when a cosmic ray entered the chamber. Blackett had also devised another way to speed up research work, by using a spring mounted diaphragm to quickly readjust the chamber back to the conditions required to observe a cloud trace
1894-5 Charles T.R. Wilson invents the cloud chamber to make small clouds in the laboratory, due to his interest in their formation and the electrical and optical phenomena associated with them
1924 Patrick Blackett uses the cloud chamber to observe the transmutation of nitrogen into fluorine, which then disintegrated into oxygen
1910 Wilson realises that the cloud chamber could be used in the task of identifying and describing newly discovered sub-atomic particles emitted by radioactive materials

What were cloud chambers replaced by?

Cloud chambers were the main type of detector used in particle physics until the 1950's, when they were replaced by bubble chambers andspark chambers. These are more sensitive and practical devics which also allow a greater range of particles, other than just ionising ones, to be detected.

Further Reading

Appendix

In order to look at why supersaturation occurs we must first introduce some equations of the first law of thermodynamics. As stated earlier, this is the conservation of energy; it 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)
U is the internal energy of system
W is work done on or by the system, where work done by is positive and work done on is negative

It is also known that work= force multiplied by the perpendicular distance:

W=F.d

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

F=PA

P being the pressure and A being the cross sectional area of the gas (i.e. it’s container). If we combine the above two equations we now have:

W=PA.d


which gives

W=PV

as an area multiplied by a distance gives a volume. Therefore 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, and the internal energy of the gas is therefore also linked to the absolute temperature of the gas.

KE=3/2NkT

Where:
k is the Boltzmann constant
N is the number of molecules
T is the absolute temperature

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

∆Q=∆ 3/2NkT+∆PV

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

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Topic revision: r15 - 03 Jul 2012 - 14:41:16 - NigelWatson
 
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