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How to Build the World’s Simplest Particle Detector

The views expressed are those of the author and are not necessarily those of Scientific American.

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In about 10 minutes, using stuff you probably already have lying around your house, you can watch atomic nuclei and elementary particles for yourself using a diffusion cloud chamber—a rudimentary particle detector. There are lots of websites and YouTube videos giving step-by-step instructions to build such a chamber, but all require some component that’s hard to come by, such as dry ice or a high-voltage power source. I’ve gotten around that by merging a cooling technique devised by Canadian high-school student Olivia Donovan with the chamber designed by Australian particle physicist and science communicator Suzie Sheehy. It’s not super-great as a cloud chamber, but it definitely reveals particles whizzing through it.

You’ll need the following:

  1. sponge
  2. rubbing alcohol (92%)
  3. clear plastic cup
  4. tape
  5. black construction paper
  6. foil cupcake liner
  7. blu-tack
  8. foil tray
  9. air duster (one of those spray cans you use to blow crumbs off computer keyboards)
  10. bright LED flashlight

The real innovation here is the air duster. The difluoroethane it releases is cold—cold enough to supercool alcohol vapor, which is what you need for a cloud chamber. The supercooled vapor will condense along the paths of ionizing particles like a tiny contrail.

The one thing you probably don’t readily have is a source of ionizing particles, but you’d be surprised how many household items are mildly radioactive, and scientific suppliers sell test sources. I bought a chunk of uranium ore from United Nuclear. (If nothing else, receiving a box from a company called United Nuclear will impress your friends.) Even if you don’t have a source, you can use the cloud chamber to see cosmic rays—energetic particles from outer space.

The plastic cup is the chamber proper. Here’s how you prepare it:

  1. Cut a piece of sponge a few inches square, soak it in rubbing alcohol, and wedge it into the bottom of a clear plastic cup, holding it in place with tape.
  2. Mount your radioactive source, if you have one, to the inside of the cup just below the rim. I found it important to mount the source to the cup rather than just lay it on the bottom of the chamber; that way, you slow down how fast the source gets coated with alcohol.
  3. Cut a circle of black construction paper to match the rim of the cup. This will provide a dark backdrop to view particle tracks.
  4. Flatten the cupcake foil, center the paper circle on it, and tape it down.
  5. Press blu-tack along the cup rim. This will be the air seal between the cup and foil.
  6. Turn the cup upside-down.
  7. Press the cupcake foil to the rim. The paper circle should be inside the cup. Smush the blu-tack into any gaps so that you have a decent air seal.
  8. Place your chamber above the foil tray to catch difluoroethane released from the spray can. I mounted the cup on a chemistry lab stand, but you can just hold it with your hand (which is better in some ways, because your hand warms the sponge, increasing alcohol evaporation).
  9. Shine the flashlight into the chamber. I got the best results when the flashlight was nearly horizontal and touching the cup to minimize reflection off the plastic.

YMMV, so play around.

To operate the chamber, turn off the room lights, hold the air sprayer upside-down, and spray the foil for a couple of seconds. Repeat every 10 seconds or so to keep the foil cold. A sign that it’s cold enough will be that ice crystals form on the outside of the foil.

Inside the cup, a mist of alcohol droplets forms almost immediately along the bottom, within a centimeter of the construction paper. If you have a radioactive source, you should start to see tracks radiating from it. If not, you’ll see a cosmic ray streak across the bottom of the chamber every 20 seconds or so. If you can’t see anything, change the illumination angle. I got significantly better results by replacing the foil and paper with a square of aluminum sheet metal covered in black electrical tape.

The spray cans are exhausted quickly. I bought a four-pack of them. To prolong the particle fireworks show, buy some dry ice—my local ice cream store sells it—and lay the chamber on a block of it.

Happy particle hunting!

Photos by George Musser

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. jtdwyer 6:52 pm 10/15/2012

    When I was growing up in NE Texas, I noticed on several occasions when humidity was high and the sky was bright but heavily overcast, I could look across the street in the shadows under the evergreen shrubs and see what looked to me like a very light rain coming down – but it was dry. I guessed then that there were some kind of difficult-to-see particles ‘raining’ down from the sky… I guess now that this was the ‘world’s simplest particle detector!’

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  2. 2. D3VIN 12:17 pm 10/17/2012

    I like how you came up with the items that a common person has in there house. who old do you think it is to try this. what made you think to come up with the items

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  3. 3. Daniel35 6:31 pm 10/26/2012

    My first “particle detector” was a ring that I got from a cereal box or from mailing in a boxtop in the late 40′s. In the dark I could look through a small lens to see flashes of light in some kind of fluorescent material. In the 50′s or 60′s I made a cloud chamber not very different from that described, but I don’t remember if I ever had a sufficiently radioactive source. I found that it helps to have something like a pin sticking up in the middle to focus the eyes where you might see the tracks.

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  4. 4. bjorn50 11:15 am 11/1/2012

    I would take issue with the use of the canned “air” duster as part of this project. The chemicals inside theses units are a dangerous and often explosive mix of gasses. Kids inhale the gasses in a growing epidemic called “huffing.” Additionally, the empty cans are classified as hazardous waste by the EPA.

    Read the label on this stuff.

    There has to be a better way.

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  5. 5. George Musser in reply to George Musser 11:27 am 11/5/2012

    @bjorn50: If you know of a better way, please let me know. Alternatives, such as dry ice and the high-voltage DC supplies required for expansion cloud clambers, require special handling. I don’t think people should be scared off doing a valuable physics demonstration because some young people are abusing the substance.

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  6. 6. idurham 12:47 pm 02/8/2013

    Hi George,

    I’m thinking of trying this as a demo in one of my classes. I’ll let you know how it goes.


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