The solar system is a hostile place, and the Earth is constantly bombarded with cosmic rays. Our atmosphere protects us from the onslaught, but when such a ray strikes an atom in the atmosphere, it sometimes shoots a shower of charged particles down towards the surface, in a process called secondary emission. With a little bit of luck and a few items you may have around your house, you could watch these particles whizzing by!

Step 1: Gather the Materials

To see these sub-atomic particles, you will need to build a cloud chamber. A cloud chamber is an experimental setup for viewing sub-atomic particles by observing the condensation trails they leave in a cloud of suspended droplets of alcohol vapor.

A cloud chamber consists of a transparent container, a warm surface, a cold surface, a felt pad soaked in rubbing alcohol and a light source.

Building a cloud chamber is very easy. In fact, that it can be made out of things that you may already have around your house. Let's look at the components one by one:

A transparent container: A transparent plastic cup is all you need. Jars or other containers from the recycling would work too.
A warm surface: A hand-warmer would work great if you had one, but I chose to use a sponge which I soaked in water and microwaved until it was nice and toasty.
A felt pad: I used a small bit of felt, but a piece of cloth, or perhaps some cotton balls would work too.
Rubbing alcohol: Nothing special here. You could also use some Jack Daniels in a pinch, I suppose.
A light source: Just any old flashlight

The only tricky part about building a cloud chamber is the cold surface. You will need a metallic surface that is kept at a temperature a few degrees below the freezing point of water. Most DIY cloud chambers use dry ice, some use an active cooling system using peltier coolers, but I had neither of these things.

After some research, I found a paper by Kyohei Yoshinaga, Miki Kubota and Masahiro Kamata that suggested that a mixture of ice and salt could be used instead of dry ice, which was perfect for me.

An aluminum heat sink, such as one salvaged from an old PC, is also required. Fortunately I had one in my junk pile.

Step 2: Paint It Black

The bottom of the cloud chamber needs a black surface for optimal viewing. Since my heat sink was a bright green, I used a can of spray paint to paint the bottom side black.

Step 3: Preparing the Viewing Chamber and the Alcohol Pad

The viewing chamber is a clear plastic cup. On the bottom of the cup, you need to affix a piece of felt. This needs to be in contact with the bottom of the cup and it needs to be glued down so that the cup can be turned upside down without it falling out.

I cut out a piece of green felt and dabbed hot glue on it. I then quickly slid it into the cup and stuck it to the bottom.

Step 4: Assemble the Viewing Chamber

The plastic cup needs to sit on the flat bottom of the heat sink. My heat sink had some raised edges, so I cut little slots in the cup for it to lay flat.

It also happened that the mouth of my cup was larger than the heat sink, so I traced the mouth of the cup onto a black plastic Qdoba plate and cut out a disc large enough to cover the opening. Out of this, I measured and sliced small pieces to close the gaps around the heat sink. I hot glued these to the cup, leaving an open space for the heat sink to fit into.

Step 5: Prepare the Ice and Salt Mix

In order to cool the heat sink down to the required temperature, I needed ice shavings and table salt. I did not have ice shavings, so I grabbed ice cubes, a cloth bag and a rubber mallet and crunched the ice myself.

I then found a plastic container large enough to hold the heat sink and filled it with the ice shavings. I added table salt in an approximate 3:7 ratio, i.e. about six ounces of salt to the approximately fourteen ounces of ice and mixed it all up.

I then pressed the heat sink down into the ice, with the flat side on top and the radiating fins deep under the ice.

Step 6: Heat the Sponge and Saturate the Alcohol Pad

For the heat source, put a small sponge in a dish, soaked it with water and heated it in the microwave for a minute and a half.

I poured some alcohol into the plastic cup, fully saturating the felt. I then set the cup on the heat sink and used some thongs to place the hot sponge over the cup (actually, I put it in a small bag first to contain any leaks).

Step 7: Illuminate and Observe!

Finally, grab a flash light and switch off the lights. Shine the flash light in at an angle and look inside carefully. Try different angles of light until you see little specks falling gently from above, like snow. Once you see them, you've found a good position to observe.

It may take a few minutes, but you'll catch little gossamer wisps out of the the corner of your eye. They are very faint, but they are what you are looking for. These are formed by sub-atomic particles as they zip by. These particles are charged and they cause the alcohol molecules to clump, condensing a trail in the alcohol vapor.

You'll know you're looking at the right thing when they form quickly then disintegrate, falling towards the bottom at the same rate as the "snow".

If you wait long enough and are quite lucky, you may see a much longer, perfectly straight trail. These are quite likely muons that were created far up in the atmosphere when a cosmic ray hit the Earth's atmosphere!

Millions pass through our bodies everyday, totally unseen and unfelt, but now you've used some ingenuity and basic science catch a glimpse of them! Congratulations!

Step 8: Taking It to the Next Level

There are several ways to improve this design. If you want to take it to the next level, consider scaling up to a larger viewing area using an aquarium and experimenting with dry ice or peltier coolers to obtain lower temperatures and better tracks. Or you could read up on how to identify different sub-atomic particles from their trails. Have fun!