Chamber Types

OVERVIEW

There are two types of cloud chambers that operates in completely different manner but both provides the same thing - to make the path of tiny invisible charged particles "visible" in the form of cloud-like tracks.

Particle tracks seen in cloud chambers are basically droplets of liquids forced into condensation from a so-called "supersaturation state". As charged particle zips through the sensitive part of a cloud chamber, it rips off electrons from the gas atoms surrounding its path and creates a trail of ions which act as "seeds" (we call nucleation spot) for the vapour to condense upon. The condensed liquid droplets can accumulate into visible sizes under appropriate lighting so we can make the presumption that charged particles must have been through those tracks. 

When cloud chamber was invented, water was used as the condensant (condensant means the liquid responsible for the formation of tracks), so as water was forced to condense on the location where particle had passed, what these scientist saw at that time was literally tiny trails of clouds where charged particle has passed, much like aeroplane contrails.

To see particle tracks, a cloud chamber must provide these so-called "supersaturation state", where the space inside the chamber is so saturated with vapour, it is extremely unstable; mere presence of dust or ions (if dust is not available) will force the vapour to condense on it immediately. There are two ways we can achieve the state of supersaturation and that leads to two different cloud chamber designs:

1. Wilson's Cloud Chamber (1899), also called expansion type cloud chamber.
2. Langsdorf's Cloud Chamber (1939), also called diffusion type cloud chamber.

Most videos on YouTube you can find about cloud chambers either DIY or exhibits features the Langsdorf-type cloud chamber as it has more visual appeal and are considerably easier to make compared to Wilson-type chambers. In this page, I provide a general history of both chambers and a table of summary at the bottom. 

WILSON'S CLOUD CHAMBER

Charles Thomson Rees (C. T. R.) Wilson was credited as the first person who visualized ionizing radiation with a cloud chamber of his design. Prior to him, people knew that if dusts were present in a chamber filled with moist air, the water vapour in the air will condense on the dusts, creating fog or momentary "clouds" inside the chamber. Wilson found that without dust, ions could do the same thing, and in 1911 to 1912, he took advantage of this phenomena and improved the technique to photograph ionising radiation from X-rays, alpha particles, fast electrons (beta particles), and gamma rays. For this, he was awarded the Nobel prize in physics in 1927.

The most popular chamber Wilson used was the one from 1912 until the 1920s. The following is a cut-away view:

Here is how it works in chronological order:

1. The chamber is a cylinder and completely enclosed. The space between A and B is the "sensitive volume" where particle tracks are to be seen. The air inside the chamber is now saturated with water vapour from F.
2. A vacuum pump sucks the air out of a spherical bulb tank, D, to low pressure. Valve C is closed at the moment, separating the air pressure from the space in D and the space below B until D reaches maximum vacuum.
3. As valve C is opened, the air below B is suddenly sucked into D. This forces the platform B to get "pulled down" suddenly, increasing the volume between AB. This sudden increase in volume drops the temperature of the chamber so much and so fast that it creates a momentary supersaturation state. Any ions present within AB will act as a "seed" for water vapour to condense on, creating cloudy tracks.  
4. The expanded volume AB is limited by a cylindrical ring of wooden block, W, and brass water container, G.
5. The cloudy trails forms quickly and disappears quickly within a second as the supersaturation state is destroyed by ambient temperature. Photograph must be taken within this time.
6. Valve I is opened, the space below B is brought back to normal atmospheric pressure. Then closed again such that step 1 to 5 can be repeated until all water vapour supply, F, is depleted.
7. The battery K, provides a DC voltage to "sweep" unwanted ions between AB just before each new expansion cycle. This will help to produce nice sharp tracks.    

Here is a video of the actual apparatus used by C. T. R. Wilson. The video was part of a museum exhibit called COLLIDER done in 2014. The cloud chamber now rests in where it was used - Cavendish Laboratory, Cambridge University:

Collider - CTR Wilson's Cloud Chamber from Street | Films on Vimeo.

From the steps above, it is clear that the operation of Wilson's cloud chamber is momentary by nature. To appreciate particle tracks with Wilson's chamber, photographs must be taken shortly after tracks begin to form because they will distort and disappear rapidly. Special techniques must be designed to accommodate such precise timing. If performed well, high quality particle track photographs can be taken repeatedly by cyclic iterations of expansion as much as the researcher desired. 

Due to the scientific impact of photographs taken with Wilson's cloud chamber, it was quickly modified by many scientists for enhanced performance to cater specific nuclear physics experiments. For nearly half a century since its invention, it was the only way to convincingly visualize the movement of nuclear particles; despite its long history of accomplishment, the Wilson cloud chamber suffered from a number of disadvantages - it was meticulous to operate. The sudden movement of piston tend to form turbulence in the vapour particularly on designs that uses rubber diaphragm; the timing of expansion, lighting, photography, and application of ion-clearing voltages has to be perfectly timed, also flash photography at that time introduced intense heat pulses to the chamber that forces the cyclic operation to slow down considerably.  

LANGSDORF'S CLOUD CHAMBER

In 1939, Alexander Langsdorf did a huge modification to the Wilson's chamber that produced successful continuous supersaturation state. The cut-away view of the original Langsdorf's cloud chamber looks like this:

The Langsdorf chamber is an air-tight cylinder, where the top part is connected to a hot bulb containing a highly volatile liquid such as methanol or isopropyl alcohol. The liquid is heated  indirectly so that it basically evaporates into high vapour pressure without "mist" because "mist" acts as nucleation spot for the vapour to condense on, which will reduce the efficiency of forming particle tracks. The high pressure, clean volatile vapour, then diffuses through a porous plate into the main volume of the cylinder.  

At the bottom of the cylinder held two layers of glass, where in-between flows a transparent coolant. The temperature of the coolant must be very low with respect to the temperature of the volatile vapour above. If the temperature difference between the lower and upper part of the cylinder is high enough (> 100 K), a small layer near the cool part of the cylinder became continuously supersaturated: vapour from the top is being cooled more than it need to condense. Any ions present within that sensitive layer will cause condensation of the vapour onto it into droplets, much like the atmosphere in Wilson's chamber during expansion, and tracks of particles can be seen if it passes through that layer. The dry gas at the bottom prevents frost that blocks the camera view (the camera shoots from below to top) as the coolant is typically at -40°C or lower.

At first glance, it would seem that Langsdorf's design is the ultimate upgrade. Finally gone the days we need to design timers and triggers to repeatedly "pulse" a Wilson's chamber! However, there are some technical challenges the Langsdorf chamber faces: 

1. The chamber must maintain high thermal gradient to sustain supersaturation. The temperature difference must be kept extremely constant to prevent convection currents in the sensitive layer.
2. The diffusing gas must be stable under gravity. With large temperature differences, this is a severe challenge that demands a vapour of light molecular mass (usually methanol) diffusing into a relatively heavy gas (CO2 reportedly works well).
3. The vapour diffusing into the sensitive layer must be free from nucleation spots. A high voltage system must be installed, to "sweep away" unwanted ions from the top half of the chamber to have appreciable tracks.   

Today, the Langsdorf type cloud chamber is by far most commonly used in school demonstration or built by DIY enthusiasts. The cold part can be generated by "dry ice" or Peltier elements or refrigerator heat exchangers while the hot part is usually a warm damp towel or electric heaters. The reasons for its popularity is simple: the parts to make a simple diffusion type cloud chamber can be extremely simple (literally just dry ice, plastic cup, and a piece of rag cloth) while more sophisticated parts are becoming increasingly affordable and available, not to mention the tracks has definite visual appeal over Wilson type chambers. The viewing of tracks is not interrupted by expansion cycles; it is easy to convince people that the tracks they saw was real-time. 

The truth is, the tracks are formed very long after the particle has passed (it does not appear to be long in our timescale but relative to the speed of the particle it is "eternal") and each time after a track has formed, the location where the track was will be temporarily devoid of vapour and must be replenished by diffusing vapour from above. This creates a momentary temporal "blind-spot" in that area. If any particle passing through the chamber intercepts those "blind" spots will appear broken. This is bad news if you want to do measurements with the tracks. Moreover the Langsdorf type chamber must be installed vertically with respect to the earth's surface. This significantly reduces the probability of cosmic rays passing through the chamber in such a way collision experiments can be conducted. For these reasons, it was not considered a practical way to study cosmic rays (the only source of very high energy particles then) and scientists at the time really just used the expansion chamber until the technology was deemed expired by the late 1950s. 

Despite its shortcomings, diffusion cloud chambers did had its use in science. After all, it was designed as an improvement to the expansion type cloud chambers. Expansion chambers are very slow when it comes to detecting particles. The waiting time between each expansion cycle can last up to few tens of minutes depending on chamber size and volatility of the condensant. Accelerators on the other hand is churning out particles faster than the working capability of expansion type chambers. So diffusion chambers came in for a short while to fill up the "technological gap" before bubble chambers - a "buffed up" version of cloud chamber - replaces cloud chamber entirely, particularly in "big science" experiments.

Here's an excerpt from an article entitled "Methods of Particle Detection For High-Energy Physics Experiments" by  H. Bradner and D. A. Glaser presented in the 1958 Second United National International Conference on the Peaceful Uses of Atomic Energy on both types of cloud chambers:

TABLE OF SUMMARY

References

1. Principles of Nuclear Radiation Detection, G. E. Geoffrey, J. W. Poston, Ann Arbor Science Publisher Inc., 1979.
2. A Report on the Wilson Cloud Chamber and its Applications in Physics, N. N. Das Gupta, S. K. Ghosh, Rev. Modern Physics, Vol. 18, No.2, 1946.
3. The Principles of Cloud-chamber Technique, J. G. Wilson, Cambridge University Press, 1951.