October 15, 2012 | 6
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:
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.
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