Cloud Chamber Background
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The cloud chamber, invented by C.T.R. Wilson in 1911, has played an important role in
many particle physics discoveries. Wilson was awarded the Nobel Prize in Physics in 1927
for this invention. The cloud chamber is a simple particle detector that uses a supersaturated alcohol layer to pick up tracks of ionising radiation. When an electrically charged
particle such as a proton, electron or muon passes through the chamber it creates a visible
track of droplets. One common type is the diffusion cloud chamber.
A schematic explaining how this works is
shown to the right. A temperature gradient
is created in the chamber causing alcohol
from a reservoir to evaporate and diffuse
from the upper, warmer area to the cold
chamber bottom. The top is at room
temperature (around 23° C) and the bottom
is cooled with dry ice (frozen C02), which
has a temperature of –78° C. As the alcohol
vapour diffuses down from the top and
cools, a supersaturated layer several cm
thick forms, where the alcohol is on the
verge of condensing into droplets
When a charged particle passes through
this ‘active layer’, it collides with the air
molecules, liberating electrons and leaving behind a trail of ionisation. The ions trigger the
condensation of the alcohol and a visible trail of alcohol droplets is formed. Photons only
indirectly create a trail when, for example, they eject an electron from atom (Compton
scattering), which produces a trail of ionisation.
A variety of different types of tracks can be created. Some are caused by alpha particles
emitted by radioactive impurities in the chamber. Because alpha particles are heavy and do
not move very fast, they deposit almost all of their energy in a very localised region and
appear as small ‘mushroom clouds’, short and dense.
Some of the tracks are low energy electrons, knocked out of their atoms by cosmic rays.
Because electrons are relatively light, they are buffeted around by collisions with air nuclei
and therefore leave jagged tracks.
The straight tracks are higher energy cosmic rays. These are mostly muons, which come
from cosmic ray showers initiated by a high energy proton when it collides with the earth's
atmosphere. Other tracks are electrons and positrons from electromagnetic showers
created when cosmic rays strike the roof of the building.
Cloud chambers can also be used to see the tracks left by electrons emitted in beta decay.
A radioactive source, such as very weak strontium-90 (90Sr), can be mounted in the sides of
the chamber and the electrons from individual decays will be seen. By placing a magnet
under the chamber, the electrons will travel in a curved path. Those with high energy travel
almost straight, and those with lower energy are more curved.
If you watch the chamber for a minute or so you should be able to convince yourself that
not all electrons come out with the same energy. This observation is what led Pauli to
propose that an additional particle is emitted in beta decay, which is now called the
neutrino (). This particle is in fact classified now as an electron-type antineutrino and what
is observed in the cloud chamber are the electrons from 90Sr -> 90Y + e- +  e .
Instructions for a simple DIY version and more information about cloud chambers can be
found at royalholloway.ac.uk/PPresources.