Fine Structure

Your Cloud Chamber

You've heard physicists tell you before that high-speed particles produced in the sun and other exotic locations are constantly zipping through your world but it's likely that you've never seen them. Are you supposed to take what these physicists say as fact without being given any proof whatsoever? Absolutely not! To prove that these "invisible" particles do exist and that they are constantly traveling through ourselves and objects all around us, we'll build a simple detector (a cloud chamber) to view them using some common items.

Let's start with our bill of materials and where you might find them:

A glass jar with a metal lid:
    We're looking for a couple things here: Visibility, a good thermal conducting surface and a tight seal. You'll be using the jar upside-down so less curvy glass near the lid of the jar is better to clearly see the particle tracks. The lid should definitely not be plastic since we want it to get as cold as possible and it should seal the jar pretty well. We're not putting any pressure in the jar but the alcohol vapor we add inside shouldn't have any place to go except in the jar. I used a glass jar from some pickles or something similar. It's the biggest thing I could find with the least amount of curvy glass - you can go bigger if you find something that still suits the requirements!

A swatch of felt:
    We're looking to cover the bottom of our glass jar with this felt. Any felt will do. I traced the bottom of the jar on to an old wool sweater that had accidentally run through the wash, cut it out and I had my felt.

Hot glue gun + glue sticks:
    This'll be how we attach the felt to the bottom of the jar. Hot glue isn't the only item we can use here but we do need something to make the felt stick to the bottom of the jar. If you're going to use something else, keep in mind that we're going to saturate the felt with alcohol and most tape will unstick after this occurs. If you ARE going to use the hot glue gun, be careful. You definitely do not want burns all over the tips of your fingers (or anywhere else, for that matter) from touching the metal end of the gun.

Black latex paint + paintbrush:
    Using a glass jar leaves us with some unwanted consequences, we'll be able to see right through the jar and the particles we're looking for blend in really well with the opaque dry ice that we'll be using. We need to beef up our viewing area with some black paint so we'll be able to see the particles clearly.

A lower container:
    We'll be setting our jar in a small container of dry ice. If the walls are too high, we won't be able to see what's happening in our jar. If the walls are too low we might not have enough space to contain the dry ice. I used an empty plastic container from yogurt. You may have to cut your container to fit well. Check the picture below for a better idea of the size of this container

Jar, Container and LidA flashlight:
    I assume you know what a flashlight is. Brighter is better. Some people suggest using an old slide projector but I find that it's easier to see the particle trails when you can move the light around to get the best viewing spot. More on this later.

Next, the important parts:

Dry ice:
    This might be the most difficult to obtain but definitely not impossible. Lots of things are shipped with dry ice to keep them frozen and then retailers just leave the dry ice to sublime away afterwards. If you catch them at the right time you may be able to walk away with a good amount of free dry ice. HOWEVER, dry ice is really freaking cold. It's sublimation point is somewhere around negative 109??F. This is easily cold enough to do some damage to your skin or anything else you might touch with it. Use tongs or gloves or some other form of protection when handling it. That said, you should check your local super markets, ice cream stores or anywhere else you can think of that gets shipments of frozen items. One of those places should be able to give you some dry ice or at least tell you where you might be able to find some. You don't need too much, just enough to fill our lower container is fine. You'll either get pellets or slabs of dry ice, both should work equally well although you may have to crush some of the slabs up in to smaller parts to fit around the lid of the jar - we'll cover this in more detail later.

Isopropyl Alcohol:
    We'll also need some pure isopropyl alcohol (aka isopropanol or 2-propanol). It's important that you find 99% (sometimes referred to as "Pure") isopropyl alcohol and not 70%. Check the local drug stores for this item. Isopropyl alcohol is not any kind of alcohol you'd want to drink and it's pretty unpleasant to smell as well so keep yourself in a well-ventilated room.
Step 1: Attaching the felt
    We have a couple modifications to make to our glass jar before it's ready to be used as our cloud chamber. First, trace a circle (or whatever shape the bottom of your jar is) in your felt and cut it out. Get out your hot glue or whatnot and secure that felt to the bottom of your jar. You might want to press the felt down to the hot glue with something so it doesn't come off. It shouldn't take long for the glue to cool and you'll be ready to move on to the next step.

Step 2: Painting the jar
    Painting parts of the jar black helps immensely in viewing the particle trails in the jar. You'll want to paint the inside of the lid as well as 75% of the way around the inside of your jar, about an inch to two inches from the top of the jar.
Painted JarStep 3: Performing the experiment
    Now that the paint on the jar has dried, drip some of your isopropyl alcohol on the felt inside the jar. Enough to make it nice and wet but not too much or you'll get a little alcohol pool on the bottom of your jar. We want alcohol vapor here, not liquid (not yet, at least). Once you've saturated the felt, seal your jar up tight and break out the dry ice. Fill the bottom container with a small layer of dry ice that your jar can rest on. Now, place the jar top-side down on that dry ice and fill in around the sides of the jar with some more small dry ice pieces. Add enough to cover up the sides of the lid, at least, but don't block your viewing window. Place the entire container with dry ice and jar in a location where it can sit undisturbed for 5 to 10 minutes. At this point it's good to be in the dark and to have your flashlight out. If you want, check to make sure everything is going well by shining the flashlight into the container. Check for a light misty rain that should be floating down from the felt at the top of the jar. This means it's working :) I should mention now that you'll probably want to move the flashlight around to obtain the best viewing angle for the mist - I found that keeping the light right next to my head works well. After the alcohol vapor reaches the bottom of the jar and has some time to cool down, you should start seeing thin lines forming across your rain mist. These are the tracks of high-energy particles!

How it works:
    As the alcohol evaporates from the felt, it falls to the bottom of the container as vapor and then cools, condensing into alcohol again at the very bottom. The vapor cools to just below it's condensation point between those two steps, making it supersaturated. At this point even very slight disturbances in the vapor will cause it to condense. The particles have enough energy to do this very thing and cause a line of condensation to form and fall in the supercooled portion of the container.

Afterwards:
    This simple experiment can be extended once you've tried it a couple times and can become much more interesting for someone with an extra interest in physics. Here are some things you could try:
Be sure to leave a comment with your experience performing this experiment, especially if you've extended it past the instructions and have found interesting results!

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