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How to Make a Cloud Chamber!

A cloud chamber is a simple device that allows you to observe the decay of radioactive materials. Learn how to build your own! While it isn't difficult to build, it does require dry ice, isopropanol and a source of radiation, all of which are commercially available.

Announcer: Frostbite Theater presents... Cold Cuts! No baloney!

Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: Today, we're going to show you how to make a cloud chamber!

Steve: Yay!

Joanna: First, you're going to need a piece of black construction paper, a pair of scissors, some sticky-back felt, a Petri dish with a lid, some isopropyl alcohol, a flashlight, a radiation source, and, last but not least, a block of dry ice.

Steve: Now, some grocery stores carry dry ice, so it shouldn't be that hard to get your hands on some. But that's one thing you really don't want to do. You really don't want to get your hands on dry ice. The dry ice is cold and it can damage your skin. So, when you handle it, make sure you wear gloves.

Joanna: Isopropyl alcohol should be at your local drug store, but what you find might not be what you want to use. The easiest stuff to come across is 70% isopropyl alcohol. That means that it's 30% water. Now, while this will work, it just won't work as well.

If you just search around a bit more, you should be able to come across 91% isopropyl alcohol. This is going to be what you want to use.

Today, however, we're going to use 99.5% isopropyl alcohol. We have this here at the Lab, but you probably won't find it at your local drug store. If you really want to use a source that's this pure, you're going to order it from a chemical supply company.

Isopropyl alcohol is flammable, so you don't want to use it near an open flame or anything else that's hot. If you've never used isopropyl alcohol before, you need to read the MSDS sheet. You can find this by googling 'isopropyl' and 'MSDS', or you can just download it from our web site.

Steve: The radiation source is something you'll have to buy from a vendor. The place we got this from is called Spectrum Techniques. And there's a very small amount of radioactive lead at the tip of the needle. It's such a small amount, you don't need a license or a permit to buy this. You just need about $35. But, this isn't a toy. Don't touch the needle and store it somewhere safe when you aren't using it. If you're a youngish kind of a person, have your parents buy and handle this for you.

Joanna: Once you have all your materials, actually making the cloud chamber is pretty easy.

First, you're going to cut a circle out of the construction paper that will fit neatly inside your Petri dish.

Next, adhere your felt to the inner rim of the Petri dish wall. You'll end up with something that looks like this.

Next, soak your felt with the isopropyl alcohol. Now, it's not a big deal if it gets on the construction paper. What's really important is that the felt is thoroughly soaked.

Place your radiation source inside the Petri dish and put the lid on top. Now, I'm going to hold it between my hands just to warm it up for a moment or two. And then we're going to place it on the dry ice.

Steve: Now, to see this better, we'll kill the lights... and light up the chamber from the side with a flashlight.

The dry ice is cooling the air inside the Petri dish. As the air gets cold, it can't hold as much alcohol vapor. This is kind of like what happens when you go 'haaa' on a cold day. You see clouds because the cold air can't hold all the water vapor that was in your warm breath. As the source decays, it sends charged particles through the air. Those particles rip electrons from the molecules of the air, leaving a trail of ions. This gives the alcohol vapor a place to condense. This is what we're seeing. We're seeing clouds of alcohol that are forming on ion trails left by charged particles that are shooting off the end of the needle.

Now, once you do this a few times, you'll notice that the tracks we're getting are from alpha particles. You can tell because the trails are fairly short and they're fairly dense. The source we're using is lead-210. And, if you look-up lead-210, it's mainly a beta emitter. It only emits an alpha particle about once every 50 million decays or so. So, the question becomes, where are all these alpha particles coming from?

Joanna: Well, when lead-210 decays through beta emission, it turns into bismuth-210, which is a pure beta emitter. Eventually, that decays into polonium-210, which is an alpha emitter. And that's where we're getting the alpha particles.

Steve: So, even though our source is labeled lead-210, it's actually a mix of three different elements. We have lead-210, bismuth-210 and polonium-210 all in there at one time.

Joanna: Thanks for watching! I hope you'll join us again soon for another experiment!

Steve: It's like the 4th of July in there!

Janna and Steve: Oooo! Ahhh!

Joanna: Oh, my!

If you do decide to build your own cloud chamber, download and read the Material Safety Data Sheets for dry ice and isopropanol. If you don't understand how to use these materials safely, do not do this experiment.

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For questions about this page, please contact Steve Gagnon.