Introduction: Detecting Cosmic Rays in a Cloud Chamber

About: I am a physics student at Wheaton College who likes building interesting physics demos and other stuff that I think is awesome!
It is kind of crazy to think that we are being bombarded at all times by tiny particles moving close to the speed of light. If you want to see  evidence of these randomly occurring cosmic rays check out the video. If you still don't buy it or want to see the vapor trails for yourself (like I did) then you should continue reading this instructable.

Step 1: Materials

Here is what I used to build my dry-ice cooled cloud chamber:

- Insulation foam (I bought some that was conveniently cut to fit between the framing boards on a house which proved a perfect width)
- Fish tank 
- Metal plate
- Black Modeling clay
- Black Cardstock
- Black Electrical Tape
- Glue (to glue the foam pieces together)
- Sand paper (to smooth the foam)
- Black Foam Poster board (this was used to create a cover for the fish tank to make the pictures and video come out nicer)
- Flashlight or old slide projector (basically you need a solid beam of light to see the cosmic rays)
- Dry-Ice (Used to cool the base of the cloud chamber)
- 91% or higher Isopropyl alcohol (70% does not work!!!)
- Water (this was poured into the clay channel on the base to create an air tight seal)

- Razor Blade
- Paint Scraper (I used it to help cut the foam)
- Yard Stick
- Sharpie
- Hot Glue (used to attach the felt to the top of the fish tank)
- Magnets (used to attach the felt to the top of the fish tank)

Step 2: Constructing the Insulated Box (for the Dry-Ice)

Essentially you need to create a insulated bo that will be filled with dry-ice. The box  needs to be big enough so that the metal base plate can sit in it snugly. I used the metal plate a a guide to cut the box out of pieces of insulation foam. I then sanded the foam down and glued everything together. When I cut the foam I would first use the razor blade and then once a small scratch had been made in the foam I would punch all the way through with the paint scraper. I am pretty sure that there was a better way to cut the foam but with the sand paper I wasn't too concerned about how it looked initially. 

Step 3: Preparing the Metal Base Plate

The metal base plate sits on top of the dry ice to facilitate a temperature transfer. One thing that is of primary importance is that the metal plate needs to be able to maintain an airtight seal with the fish tank. This maintains a high concentration of alcohol inside the cloud chamber and allows for the formation of a super saturated alcohol vapor.  In order to do this I decided that I would fill a small groove around the edge of the fish tank with water or excess alcohol. In order to create the groove I used modeling clay and just molded two tracks of clay onto the metal plate. Then to make the white vapor trails contrast with the bottom of the could chamber I attached black card stock to the bottom of the baseplate. This made the vapor trails a lot easier to see.

Step 4: Modifying the Fishtank

Essentially all that I did to the fish tank was attach a piece of felt to the bottom (which turns out to be the top of the cloud chamber). At first I just used hot glue to attach the felt to the top of the fish tank. However as you can see from the picture It sagged down when alcohol was added to the felt. To fix this I used some sets of magnets that I had lying around to stick the felt to the top of the fish tank much more securely. The felt is ultimately just used to absorb alcohol and a sponge or other kind or absorbant fabric could easily be substituted in place of felt. 

Step 5: Putting It All Together!

Now the foam box for dry ice, the metal baseplate, and the fish tank are all ready for action! 

The first step is to put dry ice into the foam box and place the metal base plate on top of the dry ice. The metal plate will probably let out a screeching sound as the metal cools but this should subside after a while.

Then pour some alcohol onto the felt until the felt is pretty well soaked in at least 91% isopropyl alcohol and place the fish tank ontop of the metal baseplate.

Then pour some extra water or alcohol into the clay channel that you made in the baseplate to ensure an airtight seal.

Now darken the room and illuminate the cloud chamber with your light source. I covered all but one of the fish tank's faces with black poster board to make it easier for the cosmic rays to be filmed but that is not necessary. Almost immediately you should see a cloud of alcohol vapor start to rain down and after about 10 min a super saturated vapor of alcohol should form. Now you should be able to observe some vapor trails towards the bottom of the cloud chamber. 

Basically the alcohol vapor is so saturated that when a cosmic ray passes through the chamber it ionizes the molecules in the air and the alcohol vapor condenses around these trails of ionized molecules. This is how the cloud chamber allows us to "see" particles that are moving close to the speed of light. To start learning more about why this works check out the wikipedia link.
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