C. T. R. Wilson shared half the 1927 Nobel Prize in Physics with A. H. Compton for "method of making the paths of electrically charged particles visible by condensation of vapour".
The following is an excerpt from the Nobel Lecture:
“Professor Wilson has been awarded his prize for the discovery of a purely experimental method, which dates back from as long ago as 1911. It is based upon the formation of clouds, which develop when sufficiently moist air is suddenly expanded. The refrigeration caused by the expansion brings the temperature to sink below the dew-point, and the vapour is condensed into small drops, which form together visible clouds. In the first stage of condensation a droplet is always formed round a nucleus. The fact that an electrically charged particle acts as a nucleus in the formation of drops could, after the discovery of the corpuscular radiations, be concluded from an experiment that Helmholtz had, long before, made when he found that a stream of vapour loses its transparency in the vicinity of electrically charged objects.
After it had become known that electricity is conducted through gases by means of ions, and that ions are formed – or, in other words, gases are ionized – under the influence of X-rays or radioactive substances, the way lay open for Wilson to follow photographically the formation of droplets around electrically charged particles. Alpha and beta particles emitted by radioactive substances ionize the gases, and their tracks are marked by a formation of droplets. A suitable photograph of these droplets then gives a picture of the tracks of the ionizing particles.”
- Nobel Lectures, Physics, 1922-1941, Elsevier Publishing Company, Amsterdam, 1965
TRACKS IN WILSON'S CHAMBERS
Until the 1960s, Wilson type cloud chamber was frequently used in schools or university demonstrations to show particle tracks produced by alpha, beta, or gamma radiation. The following text was taken from the second edition of Electricity and Modern Physics (an A-levels equivalent physics textbook) by G. A. G. Bennet published in 1974:
Cloud Chamber Tracks for Alpha Particles
"When a suitable radioactive source is mounted in the chamber, the tracks of alpha particles are strikingly shown up. Their range in air and the manner in which they are stopped by thin foils shows that the tracks are indeed those alpha particles such as we have already detected by other means. The ionization produced by an alpha particle is always very heavy; detailed measurement show that each particle produces about 3000 ion pairs per mm of its path in air at s.t.p. It is therefore not surprising that the energy of the particle is rapidly dissipated, bringing it to rest in the short distance that we observe. The path of the particles are seen to be almost exactly straight, through small deflections of 1° are fairly frequent. The patient observer, who watches the chamber for several minutes, may be rewarded by seeing an alpha particle deflected through a much larger angle (90° or more). The study of the large angle deflections of alpha particles provided Rutherford with the evidence he needed to propound the nuclear theory of the atom."
Cloud Chamber Tracks for Beta Particles
It is also possible to use a cloud chamber to observe beta rays. Again, we find a series of clearly marked tracks, and it seems that beta rays also must be regarded as particles. The ionisation along the track of beta particle is much less heavy than for an alpha particle, and the path is only thinly marked out by a line of droplets; it vanishes within a fraction of a second of being formed. But with a magnifying glass focussed on the right region of the chamber, an alert observer will manage to see the occasional track. Satisfactory observation is really only possible by photographic means. The paths of the particles are very far from straight; they seem to suffer frequent small deflections and occasional large deflections of 90° or more. The deflections become more and more frequent as the speed of the particle falls; and at the end of the track is usually very tortuous.
Cloud Chamber Tracks for Gamma Rays
A beam of gamma rays shows up in a cloud chamber in the same manner as a beam of X-rays. In this case, there is no clear line of droplets marking the path of the beam; but a number of short tracks are observed resembling those of beta particles, each track originating in the line of the beam. This is quite different from the tracks of alpha or beta particle, which only shows "whiskers" of the kind that characterize the path of gamma rays. It is difficult to observe these tracks in the conditions of a school laboratory.
A typical fan of alpha ray tracks in a cloud chamber. The tracks are almost all straight, and heavy ionization is produced along them (C. T. R. Wilson)
Cloud chamber tracks produced by a beam of X-rays entering the chamber from the right (the beam is highlighted in white through the photograph) The tracks start in the path of the beam and are identical with those of weak beta rays. Similar tracks are caused by gamma rays. Their lengths depend on the wavelength of the radiation involved. In this case, K-series X-rays from silver at 24 keV, were used, and the tracks are about 1.5 cm long. (C. T. R. Wilson; print prepared by W. H. Andrews)
In this photograph a very narrow beam of X-rays enters the chamber from the right, as indicated. The tracks observed are those of electrons ejected from the molecules of the air in the path of the beam. There is nothing to mark the path of an X-ray photon (the dotted line is artificially added for this purpose) between the source and the point at which it gives up its energy to a single electron. (C. T. R. Wilson, Proc. Roy. Soc., A, P. 104, Plate 5, 1923)
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