Maybe The Simplest Cloud Chamber

Have you ever seen a Wilson cloud chamber — a science experiment that lets you visualize ionizing radiation? How hard would it be to build one? If you follow [stoppi’s] example, not hard at all (German, Google Translate link). A plastic bottle. some tape, a flashlight, some water, hot glue, and — the only exotic part — a bit of americium 241. You can see the design in the video below and the page also has some more sophisticated designs including one that uses a CPU cooler. Even if you don’t speak German, the video will be very helpful.

You need to temper your expectations if you build the simple version, but it appears to work. The plastic bottle is a must because you have to squeeze it to get a pressure change in the vessel.

If you are wondering where to get americium-241, check out a junked smoke detector. The second video, below, has details about how to harvest it from that common source. You need something that fits in the bottle cap.

It is hard to see on the video, but different types of ionizing radiation leave telltale tracks in the chamber. Alpha particles tend to leave thick straight tracks whereas beta particles tend to be more delicate and curvy. Although in modern times these are more or less science curiosities, there was a time they were real lab equipment and they participated in the discovery of the positron, the muon, and the kaon.

If you want to try a bigger chamber, we’ve seen it done. Some chambers use alcohol and dry ice.

10 thoughts on “Maybe The Simplest Cloud Chamber

  1. Note that (in the US) such smoke detectors are legally safe as smoke detectors. Taking one apart gives you a very much not-ok source…

    Spend a bit more effort on the chamber, run it with a simpler source.

    1. Before taking an Am-241 smoke detector apart you will need to check the radioactive materials regulations of the NRC or an agreement state. You will probably need to get a specific license to do it legally.

      Better to use a piece of an old lantern mantle containing thorium, rather than the new mantles that now use yttrium.

    2. Sure, there may technically be legal issues with harvesting an americium source from a detector but it is probably the safest source your are going to easily find and it can put out a reliable amount of radiation.

      The americium is well contained on the disk and isn’t likely to flake off like uranium ores or thorium lantern mantels.

      It’s almost purely an alpha emitter (some weak gamma but you can pretty much ignore that since it’s such a rare event compared to the alpha) so it is safe unless you eat it or try grinding it off the disk.

      Smoke detectors have a clearly defined amount of the material. This is unlike something like uranium ore where the activity can vary wildly from sample to sample.

      It’s cheap since you can often find old ionizing smoke detectors at thrift stores or garage sales. I’ve picked up a few over the years for less than $5. Often more around $1.

  2. You can also do this by hooking up a big syringe to a small erlenmeyer flask (with a bit of hose stuck through a rubber stopper).
    Pull out the syringe rapidly, and the ethanol or isopropanol will vaporise for a moment.
    I tried this with uranium glass, but that stuff is not radioactive enough to make something visible in the very short sensitive period. My sample shoots out 1 or 2 alpha’s per minute.

    If you try it with radium watch hands, be very careful. The alcohol vapor might condense onto the hands, and start to spread radium paint everywhere and you really don’t wanna fuck with that stuff. The paint also can just flake off. I have had that happen, and had to very carefully pick up some flakes with a piece of sticky tape.

    The americium capsule is much safer to handle, because it does not contain any alcohol soluable parts.

    Ionisation chamber smoke detectors haven’t been sold here for a decade, but i keep some around as a handy and safe alpha (with minor gamma) source.
    If you’re looking for an affordable license free radioactive source for use in education, smoke detectors are already too radioactive. Anything over 10kBq needs a license, even if it’s a ‘domestic’ source like smoke detectors or gas light mantles.

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