Re: Why do some fluids work in a diffusion cloud chamber and others don't?

Stephen,

Sorry about the delay in replying. That is a really good question, and I had a hard time finding a good answer on what makes a good working fluid. What has made your question hard for me to answer is that every time I find a good reason for using alcohol, it turns out that water also has properties that make it suitable for viewing particle tracks. As it turns out, the original diffusion cloud chamber built by Charles Wilson (1911) actually used water vapor. This is not so surprising, since the original purpose of such research was to study cloud formation around ions in the air. Since, for example, alpha particles leave trails of ions when they pass through air, it soon became clear that the cloud chamber was a useful tool for particle research. In the years that followed, many improvements and innovations have been made in the design, including the move from water to alcohol and ethylene glycol as standard working fluids.

Here is a nicely written and very readable undergraduate paper I found on a continuously sensitive cloud chamber using dry ice and ethanol. What you can see from this report is that it is necessary to produce a steep temperature gradient between the top and the bottom of the chamber. At the top, the temperature must be high enough to produce a high concentration of vapor (so that there will be lots of vapor available to condense around particle tracks). As the vapor is formed, the temperature gradient cools it and causes it to settle towards the bottom of the chamber. Eventually, the temperature falls below the point where the vapor "should" condense to a liquid.

I put "should" in double quotes, because in reality a vapor does not just change phases (gas to liquid) on its own. It needs some disturbance to trigger the change (i.e. condensation around a handy ion). If the conditions in the chamber are stable enough, the vapor can last a long while before condensing. This condition is called "supersaturation", and it is the key to a diffusion cloud chamber.

The supersaturated vapor layer near the bottom of your chamber is the "sensitive" part of the detector, and you want it to have as large a volume as you can for best results. You also want to cool the vapor as much as possible, since the lower the temperature, the greater the degree of supersaturation, and therefore more liquid to condense around ions (making the tracks easier to see).

Now you know a bit about the design goals of a cloud chamber. So, why is alcohol a good fluid? To begin with, it has a high vapor pressure at room temperature, so you can make large amounts of vapor easily. Second, you can cool it to very low temperatures (much lower than water) without freezing. So you can put liquid alcohol in good thermal contact with dry ice (at -70 or so Celsius) without freezing it (making it hard to vaporize, and impeding thermal contact with the bottom of the chamber). This allows a steep temperature drop over a long distance in the chamber, making a thick supersaturated region.

But choice of working fluid alone doesn't decide whether a cloud chamber will work well or not. As it turns out, there are a lot of engineering aspects involved that can make or break the design. This company, for example, specialises in large-volume cloud chambers suitable for lecture halls, and able to show cosmic ray processes that a normal "table top" chamber cannot. They also have some information on cloud chambers and radioactive processes, and pointers to further reading. Andy Foland has also produced a nice link page for cloud chamber resources.

I hope this helps.