[Historical Video] Radioactivity and Neutrino Experiments involving Cloud Chambers. (1957)

 The following is an archive film from British Pathé:

According to description by British Pathé, the video starts with Hungarian title. The next scene depicts people walking past GV Academy of Science building in Debrecen, Hungary. This is followed by various shots of Professor Alexander Szalay and his colleague Julius Csiky working on experiments in laboratory. They were working on the existence of the neutrino within the atom (sic.). They are creating atomic explosions in the Wilson fog (sic.) chamber.

(Lav.) Old record suggests that material dates from around 29/03/1957.

The information was cross-checked with the webpage by Hungarian Academy of Science; It turns out Alaxander Szalay was the founding director of the institute for nuclear research in Hungary. His studies in neutrino experiments laid groundwork in modern physics. Full note on professor Alaxander Szalay's  work are available on the webpage.

https://atomki.hu/en/historic-site-in-physics

Consider a pure beta decay; conversation of momentum should result in recoil nucleus and the emitted particle part ways linearly at 180 degrees. If neutrinos exist with non-zero mass and carries some of the energy from the radioactive decay, then both recoil nucleus and the beta particle must part ways at angle less than 180 degrees to preserve the conservation. 

To prove neutrinos exist during beta decay, they chose to observe the radioisotope helium-6 in a cloud chamber because:

1. It is a pure (99.99 %) beta emitter with relatively long half-life of 807 ms. (less contamination, last longer)

2. It is gaseous. (Tracks of recoil atom can be seen and not "hidden" in solid sources.)

3. It has a large decay energy of 3.6 MeV. (plenty of energy for neutrinos to "carry away" if it exists)

The following stereo photographs seems to be the outcome of the experiment as was authored by J. Csikai: 

The shorter track is the recoil nucleus (after He-6 has decayed). The beta track, energy, and parting angle are indicated. The stereoscopic photos (24° angle setup as shown in the British Pathé video) clearly shows the "missing impulse" carried away by the neutrino.    

REFERENCE

https://www.britishpathe.com/video/explosion-in-fog-chamber

J. Csikai, "Photographic Evidence for the Existence of Neutrino", Il Nuovo Cimento, 5(4), 1957

[Historical Photographs] Cosmic Ray Observation with a Large, Randomly Operated Cloud Chamber (1939)

Here is an excerpt from a 1939 paper that describes a randomly operated (as opposed to Geiger-coincident triggered) expansion cloud chamber with some track photographs.

Description of Cloud Chamber

The essential parts of the cloud chamber used in the present work are shown in the figure below:

The main compartment A, where the tracks are formed, is cylindrical, about 30 cm in diameter and 30 cm long. Before an expansion is made the rubber diaphragm B is close to the perforated brass plate C, the valve D is shut and the reservoir E partially evacuated. To operate an expansion the catch F is knocked in the direction of the arrow. This opens the valve, and the rubber diaphragm falls on to the perforated brass plate G.

Most of the photographs were taken with ethyl alcohol vapour as condensant, the expansion ratio for which is about 1-16.

The side of C facing A is covered with black velvet, which prevents irregular motion of the gas in A during an expansion and provides a dark background for the tracks. The camera used had lenses of focal length, f, of 12 cm, and an aperture of f/5.8. It was focused on the plane J between the magnetic field coils H. With a magnification of 0.28, such as was chosen, the effective depth of focus was about 5 cm.

To avoid distortion of the tracks by convection currents it is very important that the walls of the chamber be kept at a uniform temperature (cf. Blackett and Wilson 1937), a variation of half a degree or so being the limit tolerable. 

In view of this the brass formers containing the field coils (the inside of which are within 2 cm. of the chamber walls) were kept at room temperature by passing water through the copper tubes K soldered to the side of the formers. With this precaution the distortion of the tracks (the estimate of which is described in the next section on the spectrum) corresponded in general to a radius of curvature (in the chamber) greater than 7m, i.e. to a dip in an arc 10 cm long of less than 0.18 mm. Convection currents set up as the result of the expansion are confined, during the sensitive time of the chamber, to a region close to the walls and do not give any trouble.

A mechanical method of timing the various events in the operation of the chamber proved quite satisfactory. With the arrangement used the timing could be reproduced at long intervals with an accuracy of at least 0.01 sec. 

This reproducibility is important, especially when the chamber is used for determining decay periods of long-lived radioactive substances, and it is doubtful if an electrical method of timing would be equally reliable. The chamber described here has been used for this purpose, the radioactive substance being mounted inside the chamber as shown in the figure (Walke, Williams and Evans 1939).

The interval of time after an expansion during which the supersaturation remains sufficient to cause condensation on ions—the sensitive time of the chamber—was determined by counting the tracks from a radioactive source which was uncovered at different times after the expansion, the expansion ratio used being the maximum consistent with not giving a general cloud. It was found to be about 0.4 sec, which is about 20 times greater than the sensitive time of an ordinary sized chamber about 4 cm deep.

Track Photographs

Figure 3: a and b represent an electron pair. Energy of a = 90 MeV b = 24 MeV. c is probably an electron track with energy 8 MeV not associated with a and b. e is probably a mesotron track with energy in the order of GeV. d and f are "old" tracks, probably of mesotrons with energy exceeding 600 MeV. (Magnetic flux density, H = 2200 G) 

Figure 4: a and b represent an electron pair (energies 50 MeV and 70 MeV). d is probably an unassociated electron with energy 70 MeV.

Figure 5: a is probably a mesotron track with energy exceeding 600 MeV, and b an electron track produced by a (energy = 15 MeV). c is an independent track, probably of a mesotron of 400 MeV. (H = 1000 G)

Figure 6: a, b, c, d, e, f, g are nearly parallel electron tracks, probably representing a portion of a more extensive shower. Energies exceeding 100 MeV. (H = 600 G)

Figure 7: Shower of 8 tracks. 3 have energy < 30 MeV, and 5 energy > 30 MeV. Gas is a mixture of hydrogen and air. (H = 200 G)

Figure 8: An old shower of 7 tracks. 5 are positive electrons with energy less than 100 MeV. 2 have energy greater than 100 MeV, sign of charge uncertain. (H = 2200 G) 

REFERENCE

E. J. Williams, "Some Observations on Cosmic Rays Using a Large Randomly Operated Cloud Chamber", Proc. Roy. Soc. A, Vol. 172, No. 949, pp. 194-212, (1939)

[Historical Photographs] The Amateur Scientist - Cloud Chamber (1956)

 As I was looking for my first assembly draft for AWAN, I came across an article posted by Noah's Neurosphere, which is basically an excerpt from an issue of Scientific American written by retiring columnist Clair L. Stong. The column article itself was later selected in his book "The Scientific American Book of Projects for the Amateur Scientist" published in 1960.

The original article in Scientific American contains two parts and was titled: A Computer to Solve a Problem of Mechanical Translation and an Ingenious Cloud Chambers. Here, I'm going to extract the part on cloud chamber, verbatim.

*      *      *

Among readers who requested the samples of radium offered last April in connection with the article on cloud chambers was Louie R. Hull, a physics teacher at South Side High School in Fort Wayne, Ind. He sends some photographs of alpha and beta tracks recorded in his homemade cloud chamber, one of which shows how a barrier of cellophane blocks the alpha particles. 

Beta tracks are very thin and emerge from a point left of the chamber.

"These tracks," says Hull, "were photographed with the aid of a 'plumber's friend' cloud chamber" - an arrangement assembled from odd parts from the junk box. Like the 'peanut-butter jar' chamber that you described, the plumber's friend can be constructed in a single evening. The basic idea stemmed from a chamber of the rubber-bulb compression type popular for classroom demonstrations. Although the rubber-bulb instrument is satisfactory for a visual demonstration, trouble was encountered when we attempted to photograph tracks with it. Compressions could not be reproduced uniformly, nor could we time the exposures correctly. Various alternate arrangements were tried until it occurred to us that the plumber's friend- a rubber plunger of the kind used for clearing drains-might work. In the cloud chamber it does not operate strictly as a piston does, of course, because the center of the plunger moves a greater distance than the edges. As a result the particle tracks are distorted somewhat; nevertheless we are delighted with the performance of the arrangement and have made hundreds of excellent pictures.

The side walls of the chamber are cut from a quart glass jar by the hotwire method. You wrap a single turn of iron wire (such as that used for binding brooms) around the jar tightly at the place where the cut is desired, the ends being separated by a small sheet of asbestos insulation where they would otherwise make electrical contact. The loop becomes red hot when you connect it across the six-volt terminals of a transformer or storage battery. After about 30 seconds of heating you remove the wire. Plunge the jar into cold water immediately. The glass will break cleanly at the line where it was heated by the wire. The sharp edges are then rounded with abrasive such as emery or carborundum.

The top of the chamber is closed by a plate-glass window [figure above]. If you do not own a circular glass cutter, one can easily be rigged from a wheel-type cutter available in hardware stores. Fasten the wheel end of the cutter to one end of a short length of inch-square wood so that the wheel protrudes slightly beneath the lower edge of the wood. 

Next drive a wood screw through the wood vertically at a distance from the cutter wheel equal to the radius of the desired glass disk. The protruding tip of the wood screw serves as the center point of a compass, the wheel as the other point. 

Then make an indentation with a center punch in a small scrap of 16-gauge sheet metal. You place this punched piece of metal on the glass to be cut, backing the metal with friction tape to prevent it from slipping. The punched indentation is centered with respect to the glass. Now you put the protruding tip of the wood screw in the indentation and make a circular cut in the glass with a single, firm rotary stroke of the tool. After this, if you make 10 straight radial cuts from this disk to the edge of the glass sheet, you can break away the outer pieces, leaving a disk the same size as the circular cut. You then smooth the edge of the disk with abrasive.

The glass cylinder, window and plumber's friend are fastened together with metal rings [left figure above]. The ring fittings can be built of thin sections cut from sheet metal and soldered. If a metal cutting lathe is available, you can machine them from a thick slab of stock. Rubber gaskets must be inserted where the metal and glass come into contact. Turbulence in the chamber is minimized by inserting a disk of black velveteen, supported by wire screening, between the cavity of the rubber plunger and the chamber. The radioactive sample is supported inside the chamber by a machine screw inserted through the side wall. 

The assembled chamber is supported on a wooden bracket. Its expansion is actuated by a lever mechanism, which is tripped by a motor-driven cam. The chamber is illuminated by a 300-watt slide projector, the beam of which is controlled by a shutter released electromagnetically. The camera is positioned above.

To make the compression stroke you lift the horizontal lever quickly and hook it to the vertical lever. The compression should not exceed about one third of an atmosphere, or the chamber will fill with fog on expansion. After about 30 seconds you start the motor. The cam advances until the metal arm at the top of the vertical lever drops into a notch on the cam. A spring then pulls the vertical lever away from the chamber, unhooking the horizontal lever. The plumber's friend then springs to its original shape, accomplishing the expansion stroke.

An electrostatic 'clearing' field is applied to the chamber automatically during the compression stroke by a microswitch actuated by the horizontal lever. The field is removed by the switch and the leads to the chamber are short-circuited automatically at the end of the expansion stroke. Similarly, the motor-driven cam is equipped with switches for operating the projector and shutter release in sequence. Exposure time is fixed by the tension of a rubber band hooked to the shutter.

The proportion and amount of liquid in the chamber are not critical. Good tracks form with either 180-proof grain alcohol or rubbing alcohol as it comes from the bottle. Performance is influenced by room temperature, however. Above 70 degrees Fahrenheit results are improved by diluting the alcohol slightly with water-say 15 drops of alcohol to two drops of water. It should be kept in mind that fog results from too much liquid as well as from over-expansion. The chamber rarely requires more than 20 drops of liquid. Don't expect to see tracks during the first few expansions. The liquid must have time to evaporate.

Beta tracks, being thin, are more difficult to see than alphas and appear best when the alphas have faded. It is interesting to investigate the penetrating power of particles through thin sheets of various materials. The chamber also enables you to experiment with various other atomic phenomena. If you substitute a freshly polished needle of zinc for the radium source, for example, you will see beta tracks shoot from the point These are photoelectrons released by light shining on the metal. The number of photoelectrons ejected will increase immensely if the chamber is illuminate by an arc lamp shining through a window of sheet quartz or of a clear plastic that transmits ultraviolet light readily. Such modifications of the chamber enable you to investigate the photoelectric properties of many substances a well as other forces that disturb the atomic structure of matter.

CROSS REFERENCE

C. L. Stong, "The Scientific American Book of Projects for the Amateur Scientist", Simon and Schuster, 1960, p.p. 314

[Historical Photographs] Photography Set-up for Precision Measurement of Nuclear Transmutation by P. M. S. Blackett

By the mid 1920's Blackett knew nuclear transmutations involving alpha particles can be observed directly through cloud chambers. In his previous experiment involving nuclear collisions, the tracks of interest were "forked alpha tracks"; he knew it was not easy to photograph them as the occurrence is considered very rare. Since it is not possible to "tweak" the probability of nuclear events without jeopardizing track quality, one has to take many photos in order to find one photograph containing a "forked track". Hence, there are some considerations in experiment setup - particularly on photographic methods - to optimize data taking and measurements. Improvements in this regard not only saves cost of photographic plates (or films), it also saves time in analyzing them.

The objective of optimization was:

1. Take as little number of photos where each photo contains as many alpha tracks as possible. This will help to reduce the processing and analyzing time. Do keep in mind that development of photographs with a dark room was a painstaking process back in the 20s, and analyzing photographs was done literally by rulers and eyes.


2. HOWEVER, the number of alpha tracks per photo cannot exceed a certain value of which would degrade the quality of collision / transmutation tracks if-and-when it happens. Imagine having too many alpha tracks within a given space inside the chamber, any forked track may be "covered" or distorted by other tracks of no interest.


3. The tracks should be as defined as possible. That means it must be all sharp in focus, and as thin as possible.


4. Photographs must be taken in such a way, measurement of scattering angle is meaningful. That is to say, with measurable and diminishing parallax error.

The solution was:

1. Use a pure alpha source, preferably mono-isotopic. The emissions were also collimated in such a way alphas are "fanned out" in a plane parallel to the view port of the cloud chamber.


2. Following the premise where two approximately parallel tracks is considered resolved if their physical separation is at least twice the width of an alpha track, then Blackett worked out the optimal number of tracks for a parallel beam of 1 cm width is 42. Some of the presented photos in the papers seems to contain more resolved tracks per photograph because of admitted bias.


3. Use a mixture of gases. For example, if nitrogen was intended as the target for alpha collisions, a mixture of X:Y:Z for nitrogen:oxygen:hydrogen is used. Both oxygen and hydrogen act as diluent to the target gaseous atoms, while specifically, oxygen appears to make alpha tracks thin while hydrogen increases the length of alpha tracks.


4. In order to measure the scattering angles with any degree of accuracy, two cameras was used, positioned perpendicularly to the cloud chamber. Both camera shutter was triggered simultaneously to get a stereo photograph pair. With two photos taken of known angle, it is able to work out the effect of parallax (solutions from trigonometry) and hence accurate determination of angles. Details on this will be posted shortly.


5. In order to take clear track photographs on all tracks, The plane of the lens is not perpendicular to the optical axis of the camera. It is tilted in such a way that all alpha tracks in the chamber is sharply focussed.

The following was Blackett's instrument set-up, and the resulting improved quality of alpha tracks:

The double cameras were mounted 90 degrees from each other. The chamber itself, at the "junction" of the camera lens tube was covered by a wooden black-box so a completely dark room was not necessary. The shorter silvery tube below the lens tube is a mercury arc lamp, which acts as a flash whenever the cameras are taking photos. You can clearly see the gas manifold too, showing the inlet control valves of three gas mixtures.

Using the double camera set-up with tilted lens, choice of radioactive source, and appropriate gas mixture in the cloud chamber; neat photographs of alpha tracks can be obtained. All tracks are pencil thin and crisp clear.

The alpha tracks are so fine, the limiting resolution now turns to the grain of the photographic plate. Here you can see two tracks are resolved at less than 0.1 mm physical separation. This is a negative photograph, so the tracks appears black. 

REFERENCE

P.M.S. Blackett, "On the design and use of a double camera for photographing artificial disintegrations", Proc. Roy. Soc. A, Vol. 123, 792 (1929)

[Historical Photographs] First Published Photographs of Alpha Particle Collisions

P. M. S. Blackett published in 1923 the curious case of "forked tracks" when alpha particles are allowed to travel in atmosphere as seen in a cloud chamber.

Alpha particles typically travels unimpeded under normal circumstances and leaves thick straight tracks. Occasionally some alpha particles comes close enough with the nucleus of an atom in air, causing it to deflect elastically, the "collision" process ionises and transfers momentum to the target nucleus causes it to leave tracks in the cloud chamber as well.

It is this reasoning that the forked track was interpreted as collisions of alpha particles with nucleus of an atomic nuclei.  Most of the nucleus the alpha interacted with are those of nitrogen or oxygen atoms as they both constitutes about 99 % in air.

Using a stereoscopic pair of cameras, he was able to work out the precise scattering angles of both incident and recoil particles; from the ratio of the angles he's able to confirm the masses of the particles involved. 

In many of the following photos, the shorter Y-shaped branch is caused by the massive recoil nucleus as it has a shorter range and stronger ionisation while the longer Y-branch is the deflected alpha track.  

The following were some photographs and captions from his paper:

Original caption: This shows the collision of an alpha particle with an oxygen atom, φ = 76° 6',  θ = 45° 12'

Author's note: Both left and right tracks are essentially the same. They are taken by two cameras oriented perpendicularly to the cloud chamber (stereo photography) in order to accurately measure the scattering angle. Mathematical groundwork from the paper suggest that if the recoil particle is more massive than the incident particle, the scattering angle between both particles will always add up more than 90° - which is evident in this photo.

Original caption: Collision with an atom of hydrogen. φ =  9° 21', θ = 65° 39'

Author's note: The hydrogen recoil nucleus is essentially a proton and is 4 times less massive compared to the incident alpha particle, so it is basically "kicked forward", and the sum of scattering angle is less than 90°.

Original caption: Collision with a helium atom. Although they do not appear so, the two parts of the track are nearly equally inclined to the stem. The sum of the two angles is 89° 45' - it should be 90°. (The curvature of some of these tracks is a very interesting phenomenon, now under investigation.)

Original caption: Two rather unusual forks due to collisions with air atoms.

REFERENCE

P.M.S. Blackett, "The Study of Forked alpha-ray Tracks", Proc. Roy. Soc. A., 103, p.p. 78, (1923)

[Historical Photographs] The First Photographs of Particle Tracks in a Cloud Chamber

C.T.R Wilson was the first person reported seeing and photographing "cloud tracks" formed by ionising radiation. The following photos are from the historic paper published in 1911.

His cloud chamber has a diameter of about 7.5 mm, with a height of approximately 4 to 5 mm before expansion, and 6.2 mm after. The ion clearing field was only 8 volts, which give a field strength about 16 Vcm^-1. 

Visual observation of the tracks was through a Nernst lamp (an early form of incandescent lightbulb) while photography was done with a specialized flash tube using Leiden jar arc discharges in heated mercury vapour at atmospheric pressure. 

Alpha tracks emanating from radium-tipped spinthariscope source.

Wilson noted in his paper that this photograph did no justice to the beauty of the actual cloud chamber tracks. "The cloud condensed on the ions, while varying infinitely in detail, was always of the same general character." 

Tracks due to cloud chamber exposed to X-rays. It is interesting to note that scientists at the time was still figuring out the nature of X-rays.

REFERENCE

C. T. R. Wilson, "On a Method of Making Visible the Path of Ionising Particles Through a Gas", Proc. Roy. Soc. A, Vol. 85, 578, pp. 285

[Historical Photographs] First Recognizable Cosmic Ray Particles in Cloud Chamber Photographs

Among photographs taken in a small horizontal cloud chambers, particle tracks with little deflection in magnetic field of 1500 Gauss was recorded. These tracks are not associated in the direction with the radioactive source under investigation, and they appears to enter the chamber from elsewhere. The author identified these tracks as cosmic rays and estimated the horizontal flux to be 1.2 per square centimetre per minute. 

In the image above, false colour (green) shows the tracks of a relatively slow cosmic ray electron which traversed a considerable distance in the illuminated part of the chamber. The measured momentum of this particle is 7.3 MeV/c

In the image above, false colour (green) shows the tracks of a single cosmic ray particle which was not significantly deflected in the magnetic field while beta particles from a radioactive source bends so much the curvature is effectively a circle. 

In the image above, false colour (green) shows the tracks of a pair of undeflected and almost parallel tracks dipping sharply relative to the plane of the chamber. These were no doubt members of a cascade.  

Reference

D. Skobelzyn, Leningrad, Z. Phys. 54, 686. (1929)

[Historical Photographs] Alpha Particle Scattering by Atmospheric Atomic Nucleus

If one has the patience looking into a diffusion cloud chamber containing an alpha source for longer than a few minutes, it will take an average about 10 to 15 minutes to observe something strange:

Alpha particles moving in a cloud chamber normally produces thick, straight tracks. On rare occasions, you may observe a Y-shaped track due to alpha "collisions" with an atomic nucleus. Depending on the medium which the alpha particle travels, the chances of collision varies. These collisions are mostly elastic, losing less than 1 % of its kinetic energy. 

If we assume the collision event is elastic; by conservation of linear momentum, measuring the scattering angle of both tracks at the collision centre allows us to predict the mass of the particle which the alpha particle collided. 

Such was the experiment conducted by P. M. S. Blackett back in the early 1920's. Here's the photograph from his paper: 

In these two pictures, the cloud chamber was filled with air, and the alpha particle collisions were probably nuclei of nitrogen or oxygen. Since they are more massive than the alpha particles, the emerging tracks are more than 90 degrees, and some alpha particles may backscatter (shown in (b)) as was demonstrated by Rutherford's gold-foil experiment. 

Exact measurements of scattering angles using stereoscopic cameras determines the mass ratio as 1:4 thus confirms the analysis of the event. 

Reference

P.M.S. Blackett, Proc. Roy. Soc. A, 103, 78, (1923)

Historical Photo: Alpha Particles in Cloud Chamber in Strong Magnetic Field.

Alpha particles does not bend easily under magnetic field due to its low charge to mass ratio. To show appreciable curvature in cloud chamber tracks, a (very) strong magnetic flux density of 4 T was used by P. L. Kapitza:

The magnetic field is directed perpendicularly out of the screen. Notice the shorter tracks curves more strongly than longer tracks. One of those tracks shows large angle deflection near its end, presumably the result of a close nuclear encounter. 

Image from: P. Kapitza, "α-ray tracks in a strong magnetic field", Proceedings of the Royal Society A, 106, 622, (1924). Text adapted from G.A.G. Bennet, Electricity and Modern Physics, second edition, 1974.  

Wilson's Cloud Chamber Findings and the 1927 Nobel Prize

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)