Cosmic Ray Detection @Home

Many of us are familiar with pictures from bubble chambers displaying the tracks of subatomic particles interacting. But what many people don't realise is that you can get similar results at home by spending about $40 on materials that are easy to obtain. What's more, I am one of those incompetent people who has no talent for anything practical and little attention span for anything without an instant reward and yet I managed to get succesful results. This means that if you have any interest in seeing the tracks of subatomic particles in your own home then I can almost guarantee positive results in a few hours (including time spent shopping) whoever you are. Before discussing anything there is one potential showstopper. You need to obtain dry ice. If you think you can, read on. If you think can't then check the Dry Ice Directory to see if somewhere near you sells it. You may even be able to obtain it by mail order.

Some Science.

Now that's out the way we can get to the science. Fast moving charged subatomic particles leave a wake of destruction behind them as they travel. They knock electrons off from atoms leaving them electrically charged. These charged atoms then attract other nearby atoms. If this wake passes through a vapor that is just on the verge of condensation these little groups of atoms can provide a convenient location for other atoms to land on eventually resulting in the formation of droplets of liquid. So if we can
  1. Create such a vapor and
  2. Encourage subatomic particles to travel at high speed through it
we might expect to see trails of liquid droplets form just like the trails left by aeroplanes as they travel through the sky.

Let me deal with the second point first. For that we need do nothing. Universe contains many particles traveling at high speed as a result of various types of event such as supernovae. These are known as primary rays but miles of atmosphere above our heads shields us from these. But when these particles collide with the atmosphere they send a rain of secondary particles down to ground level. The most common type of secondary particle that reaches ground level is the muon. Muons have a short half-life, around 2.2 microseconds, so you might not expect many to reach ground level. But they travel at close to light speed so that because of time dilation they actually last quite a bit longer. As a result of this, in any volume of space that isn't shielded by many miles of Earth we can expect an abundant supply of suitable particles for experimentation. So you're half way to success now without actually doing a thing.

For the first part you'll need the following:


  1. A jar with an airtight metal lid. (I don't think mine was 100% airtight.)
  2. Isopropyl alcohol. (As pure as you can, e.g. 99% pure from Long's Drugs).
  3. Some black card (as nonreflective as possible).
  4. Some glue.
  5. Various tools like a sharp knife, scissors and heavy duty gloves.
  6. A bright localised light source. (I used a desk lamp.)
  7. Some cloth. (The darker the better. By the way, mine was bright pink.)
  8. And of course some dry ice (mine came from an AM/PM gas station.)

What we're going to do is create a temperature gradient in the jar while it has a little alcohol in it. Somewhere between the hot and cold ends will be a region where the vapor is supersaturated. That's where we'll see the tracks.

Beware.

Dry ice can be dangerous. Transport it back from your dry ice store in a cooler. (I didn't, I threw it in a plastic crate, but that's not recommended). Don't handle it directly. I actually used some heavy duty gloves left over from stripping paint to deal with it. Even things touching the dry ice need to be treated with care. I managed a slight 'burn' from handling the jar lid after it had been in contact with the dry ice. Also, if you have this thing in an enclosed space (such as your car) it's giving off carbon dioxide all the time, and in theory this could asphyxiate you. Fortunately, humans have mechanisms built into them to detect this gas - if there's too much in the air you'll start feeling out of breath and may find yourself breathing harder. So work in a well ventialted area. Be very careful. I really don't want to be held responsible for any accidents and this is all at your own risk.

The Apparatus.

Scrub the label off the jar. Cut a circle of cloth that will be placed in the bottom of the jar. It should be of the same diameter as the base of the jar and glued inside. This will form a small reservoir for alcohol. The procedure is simply to wet the cloth with the alcohol (I put a few cc of alcohol in the jar and merely tossed the excess out of the window leaving the sides and base still wet - there's no need for measurements), close up the jar tightly and place it lid down on the dry ice. Note, it's important that the lid be metal in order to conduct heat easily, otherwise the jar will stay warm inside. If this setup is left for around 15 minutes tracks will probably start forming. Easy as can be!

A render of the equipment sitting on dry ice showing openings and lighting from left.

But, you won't see anything. Now comes the difficult bit - the lighting. To see the tracks you need to light the droplets from the side so that they have a high contrast relative to the background. What you do is wrap the jar in the black card. Then cut a window on the left (say), about 2cm from the bottom (ie. the lid, because the jar will be upside down at this point), maybe 3cm wide and 1.5cm high. You then cut a window at the front that is of the same dimensions but whose base is at the same height as the top of the left window (I'm now still thinking of the lid as being down). Work in a darkened room and arrange the light source to shine in from the left and view from the front. At this point I still had problems. Total internal reflection in the glass of the jar can carry light right round the jar and the rim of the jar itself glows. So what I did was make a small collar of black card which went on the inside to hide the rim and place a small 'screen' of black card against the back side of the jar (again on the inside). Any arrangement that provides a black background with as little light as possible reflected will work. (I still had reflections because everything was wet with alcohol but you can't have everything perfect.)

If you do all of this and place the jar upside down on the dry ice you should see a region near the bottom of the jar with constant precipitation. When I first did this it took me almost an hour to see this as I had no idea what to look for. You expect to see a constant 'rainfall' of tiny tiny droplets starting a few cm from the bottom of the far and rapidly falling down. You can now play with adjusting the light to get the drops as bright as you can against the background.

The detector opened up. Note the front viewing window and shield from the glare of the lamp.

Particle Tracks

After around 15 minutes you should start seeing tracks. (I don't know why you have to wait - something happens to the vapor and tracks don't seem to form before then.) In my jar I was seeing several events a minute. The trick is to be able to focus your eyes on the droplets but with a good event the track should be visible to even casual observers looking through the viewing port. Sometimes you'll see long tracks extending all the way across the jar and sometimes short tracks traveling down through the supersaturated zone. Note that they are merely reflecting light from your lamp so lighting is important. Note also that they are just droplets that are larger than the surrounding ones. They won't hover in the air, they merely fall down to the bottom of the jar within a fraction of a second of forming. If your lighting is good you'll see the entire track illuminated as it falls.

If you're really lucky you'll see more than just straight tracks. Occasionally a low energy particle will come wandering through. If the energy is low enough then each time it wanders by a nucleus in the air it won't be able to barge straight through but will be deflected. What you'll see is a short randomly shaped track. If you're really lucky a high energy particle will whack straight into a nucleus in the middle of the jar and be deflected. What you'll see then is a sharp angular turn in the track.

Photography.

I'm no photographer. I used my Sony DSC F505V digital camera to take pictures. The camera was more or less in its default state (with no flash) except that I had to manually focus on the center of the air in the jar. Note that the tracks are much more visible to the naked eye which is much more sensitive than the CCD in my camera. On seeing her first one my wife exclaimed "what was that thing like a bolt of lightning that just flew across?"
Ray successfully detected

Advanced Options.

If you have a really powerful magnet hold it by the jar. If any of the particles you are observing are charged (eg. muons or beta rays) then their paths will be curved. In you know the strength of the magnetic field you can even compute the charge to mass ratio of the particle. I used some magnets pulled out from an old hard drive. I couldn't actually see any deflectioin myself so your mileage may vary.

We did just so happen to have some of a radioactive thorium compound obtained from ebay. It was kept inside a small vial for safety which meant that the glass blocked much of the radiation. In particular it blocked the alpha rays which actually produce the best tracks (apparently). Nonetheless, it still produced beta rays. We placed the vial on the side of the jar (taping it in place) and after our eyes adjusted we could see frequent short wispy tracks. Many were formed every second though they weren't easy to see. However, radioactive material is dangerous, really dangerous. You need to know exactly what you are doing here. I trusted a friend of mine who brought the sample round and we used a geiger counter to check radiation levels.

Conclusion.

So there you have it. With a minimum of effort you can view the tracks and interactions of subatomic particles. And remember: this is an experiment, so experiment. Try more alcohol, less alcohol, different lighting, adjusting the temperature gradient in various ways or putting your old radium based fluorescent watch display that you no longer need in the jar. Have fun!

References. There are numerous descriptions of similar apparatuses on the web. This was one of my main sources. I originally wanted to build the Berkeley Lab detector but I soon realised it was beyond my ability and budget! There is also a recent Scientific American article and not so recent ones from April and December 1956.

Contact

Feel free to email me (Dan Piponi) at stirfry (at) sigfpe.com.
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