Cloud chambers are what got me into a carrer in High Energy Physics. I've been wanting to make one myself for quite awhile. There are many sets of instructions on the web, all a little frustrating,
- Quarknet/Fermilab - they want you to build your own aquarium! Instructions look complicated and it takes a week for the chamber to cure!
- Symmetry magazine has a nice article with a movie and a nice explanation of the different tracks you'll see, but lacks specifics for building (e.g. how big of a fish tank? What lid?)
- Instructables has one that uses a Peltier cooler instead of dry ice. Very cool (ha!) but complicated and expensive. You used to be able to buy an assembled chamber from a company that is now considering selling kits.
- Jefferson Lab has a nice video of making a very small cloud chamber. It's small enough that they recommend you buy a radioactive source to really make it work. I don't want to buy a radioactive source. Note that Jefferson Lab has a truly great series of science videos and demonstrations online.
- Physics super-star Brian Cox (but see here) has a clip from his BBC TV show about cloud chambers. Very nice explanation on what's going on and how cosmic rays viewable with the cloud chamber make DNA mutations, but few details on how to build the chamber itself.
I finally found a video and paper from IOPScience for a plastic fish tank chamber with instructions that were quite detailed. The whole thing looked very easy to put together. The paper is behind a pay-wall (sigh), but below I'll describe the Cloud Chamber my daughter and I built and the parts we used.
- Clear plastic fish tank. I bought this one online from Carolina, a biological supply company. Dimensions are 7-1/2" W × 11-1/4" L × 9-1/4" H and it costs a little over $10. It came to my house within a few days of ordering. Amazon doesn't seem to carry a plastic fish tank this size.
- For the base, I bought this 9x13" small baking pan from Amazon. It cost about $9.50.
- You need black felt with an adhesive back. I got a sheet from Amazon. $4.60.
- Isopropyl alcohol is needed. The kind you buy at the drug store (usually 70%) will have too much water in it. 90% supposedly works, but 99% is best. I got a 16 oz bottle of 99% isopropyl alcohol from Amazon for $8.70
In principle, the above and some dry ice (see below) are all you need. I got some extra stuff, all from Amazon, that have been helpful,
- A wash bottle is convenient for squirting the alcohol onto the felt. $5.09
- 2% Thoriated welding rods are a tiny bit radioactive and are a great source to put in the cloud chamber (the IOPScience article/movie uses them). I bought a pack of 10 for $11.49. Not sure what to do with the other nine welding rods. You can't buy just one. I guess I need to learn welding.
- Blue tack or poster tack is great for holding up the welding rod
- Good gloves for handling the dry ice. These are now apparently out of stock.
- The movie/article uses black duct tape. I didn't really need any.
- The movie/article uses the lid of the fish tank as a snap-in base and they show how to cut a big hole in it. The lid that came with the tank from Carolina was impossible to cut (it melted with a rotary tool and splintered with a saw). We determined that we didn't really need it and discarded the lid.
- Cut the felt into 1 inch wide strips and stick it on the wall, towards the bottom, around the inside of the tank. Note that we'll use the tank upside down, so from now on I'll describe the tank in that configuration -- stick the felt on the wall towards the "top" of tank. See the picture
. - You don't want to put dry ice directly on your table, so prepare a base. I had a box from Amazon with length x width 19" x 12.5" (big enough to fit the baking sheet). We cut the bottom off with a box cutter so it is 2.5" high. In my basement I have some 1" thick foam board insulation (Foamular 150 - Home Depot has it). We cut it so we could line the bottom with 1" foam board, leaving 1" open around the edge so we could cut 2.5" high strips along the walls. We then taped over the gaps with duct tape (we probably didn't need to do that). We had an extra strip, so we put that in too. Here's a picture,
The hardest part of this whole project was figuring out where to buy dry ice. Very few stores carry it (e.g. not Walmart, Costco, Target, Jewel supermarkets, nor Oberweis ice-cream). It turns out Meijer sells dry ice and they'll actually have it if you don't go on a holiday like Memorial Day. I'm not sure if they get deliveries on the weekends. You'll need at least 5 lbs. To find dry ice in your area, look at Store locator. Contentintal Carbonic has several dry ice stores as well. In Illinois, you need to be 18 years of age or older to purchase dry ice (Meijer checks).
There are some safety issues. Probably the most dangerous item is the dry ice, which is carbon dioxide in solid form. It sublimates (transitions directly from solid to gas) at -109 F (-79 C). It is very cold and must be treated with care. The Isopropyl alcohol can also be somewhat dangerous if handled carelessly. See below for hazzards.
- Hand protection: Dry ice is super cold and is dangerous to touch with bare hands. Use thick gloves. See above for some that I bought. Certainly don't eat dry ice!
- Eye protection: You don't want dry ice or isopropyl alcohol in your eyes. Wear eye protection when squirting the alcohol into the felt and when breaking up the dry ice
- Poison: For goodness sakes don't drink the isopropyl alcohol. It's not the kind of alcohol that makes you feel good. It will make you very sick or worse.
- Flammability: Isopropyl alcohol, especially at 99% concentration, is very flammable. Don't run your cloud chamber by candlelight.
- Radiation: The thoriated rods are very safe to handle and have so little radiation that they aren't a danger (see here for more information). You can buy them freely on Amazon with no restrictions at all. That being said, you should not eat them or grind them down or do other stupid things.
- Hypercapnia: Dry ice sublimates rather quickly to gaseous carbon dioxide. If you keep the block with you in a confined space, you could suffer from Hypercapnia - elevated level of CO2 in your blood - probably not a healthy thing to do. You can avoid this problem by using common sense and making sure you use the dry ice in a well ventilated or large room.
- Other: Perhaps there are many other safety issues not discussed here (e.g. dropping a five pound block of dry ice on your foot would not be fun). By using these instructions you agree that I am not responsible for them nor for you if something happens.
- If you wish, with the blue/poster tack, prop up the thoriated rod on the baking sheet so it is a little above the sheet. You may want to put it in at a small angle to test how high the vapor layer goes.
- Break up the dry ice into chunks (a hammer and screwdriver are useful - wear eye and hand protection) making a level surface in the tray. See this picture
- Put some isopropyl alcohol in the wash bottle and squirt it onto the felt all around the tank. You want the felt saturated, but not dripping. Wipe up any excess alcohol. Use eye protection.
- Put the fish tank upside down (opening on the bottom) on top of the baking sheet and put the baking sheet on top of the dry ice. It will make some neat noises as the baking sheet shrinks slightly with the cold dry ice.
- You may want to put a heavy book on top of the fish tank, just to hold everything down and ensure the fish tank is in good contact with the baking sheet (no gaps) and the baking sheet is in good contact with the dry ice. If there are gaps between the fish tank and the baking sheet, you can use duct tape to cover them. But when the tank gets cold the duct tape won't work well. We used duct tape at first, but didn't really need it.
- Turn off the lights
- Wait - it took about 15-20 minutes for things to cool down enough before we started seeing tracks.
- You need a strong light source. The LED on my iPhone 6 on medium strength worked great.
Here are two pictures of the completed cold cloud chamber.
See https://www.youtube.com/watch?v=cFG3P-VRK4E for a movie of the cloud chamber in action!
The isopropyl alcohol evaporates and the vapor falls from the felt to the bottom of the tank. The bottom is very cold and so you get a supersaturated cloud of alcohol vapor at the bottom of the tank. When a subatomic particle goes through the cloud, it ionizes the air (strips electrons off of atoms) along its path. The alcohol like to clump onto the resulting ions forming droplets, thus making a track that you can see. Cool! You aren't seeing the subatomic particles directly - instead you are seeing them affect the vapor in the cloud chamber. Still cool!
See the Symmetry magazine article for an explanation of the different tracks you'll see.