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The Cloud Chamber
"An apparatus to render visible the tracks of swift electrified particles"

Wilson's 1911 Cloud Chamber
Wilson's 1911 Cloud Chamber.
The diameter of the chamber is 16.5cm, depth 3cm
Diagram of Wilsons Cloud Chamber
The movable piston is suddenly lowered by opening the valve c and so connecting the vacuum chamber d with the part of the apparatus beneath the piston.

As the nineteenth century drew to a close, a Scotsman, C.T.R. Wilson, was intrigued by the nature of mists. He initially had been struck by the lovely colouring of the sunlight seen through a Scotch mist. His first steps were to artificially produce a mist, fog or cloud, so that he could observe and experiment. He did this by the adiabatic expansion of air saturated with water vapour.

Using his first apparatus (photo), constructed in 1899, he had determined that adiabatic expansion ratios of 1.25 or less caused no condensation at all in clean air. With expansion ratios slightly higher, the condensation was as a rain of tiny droplets. Above 1.38 the rain became a heavy fog.

X rays had recently been discovered and Wilson decided to see if fog formation was influenced by the rays. He discovered that between the expansion ratios 1.25 and 1.38 the presence of X rays considerably increased the amount of tiny rain droplets.

By 1912 Wilson had shown that radiation caused the production of ions in the supersaturated atmosphere. The ions acted as condensation nuclei, and cloud tracks formed along the path of ionising particles. Wilson also determined that positive ions do not become effective as condensation nuclei until the expansion ratio is 1.31.

Quoting Edward Neville da Costa Andrade 1929 -

"The apparatus has been variously modified by other workers since: for instance, Shimizu, who used a chamber only 6cm in diameter, found that perfect tracks could be obtained when the piston was given a reciprocating motion by a simple mechanical device. The frequency of the motion may be as high as 3 oscillations per second, which permits a large number of photographs to be rapidly taken. This form of apparatus was used by Blackett to obtain the picture...."

Transmutation photographed by Blackett

A single alpha particle from the left collides with the nucleus of a nitrogen atom. The forked track shows the results, with the newly created oxygen nucleus going off in one direction (thick track), and a proton going in the other.

Diffusion Cloud Chambers

By their very nature, adiabatic expansion cloud chambers are momentary in operation. Diffusion cloud chambers are continuous. Using a temperature inversion within a chamber, a layer of supersaturation is formed by diffusion. Usually solid carbon dioxide (dry-ice) is used to cool the lower plate of a chamber. A diagram of such a chamber, with photos of unusual tracks due to the fission fragments from californium 252, can be found here.

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