the top of the case upside down and drop the felt
inside. Now comes the tricky part — fastening the
fabric in place. You can’t simply use glue, cement,
or epoxy because alcohol eventually dissolves
virtually every adhesive.
A better solution is to pin the felt against the top
using a wire screen. Cut the screen so that it is 2-3
cm (about 1") wider than the felt, and use a pair
of pliers to bend down the four sides. Press the felt
down, and then delicately staple the wire frame in
place using ordinary office staples. (Figs. B and C)
Finally, to block out all of those track-destroying
air currents, you need to seal all of the wood/glass
joints with silicone. Run a bead of caulking along
all of the joints inside and out, and then spread the
silicone smooth with your finger. Let the silicone set
up overnight before testing your chamber.
That’s it. You’re done.
ENTER THE SUBATOMIC UNIVERSE
Place the bottom of the chamber directly on top of a
block of freshly cut dry ice (Fig. D). You want the
entire bottom surface to come to the same temperature. (Any variation will cause air currents to
flow inside the chamber and obliterate your tracks.)
So if the chamber is larger than the block of dry
ice, you’ll need to create a tile of four blocks, and
then center the chamber on that. As before, wrap
exposed dry ice with a towel to prevent smoking.
NOTE: If your supplier gives you irregular
hunks of dry ice, you’ll need to create a dry
ice and alcohol bath. First, wrap the dry ice
chunks in a towel and pulverize with a hammer.
Next, dump the pulverized pieces into a plastic
kitchen trash bag. Lay the bag on top of a
doubled towel to provide insulation, and pour
in several cups of isopropyl (rubbing) alcohol.
(You’ll waste less dry ice if you chill the alcohol
in your freezer first.) Then wet the top of the
bag with alcohol to provide a conductive seal,
and press the bottom of the chamber onto
the bag. This will chill the chamber nicely and
avoids the mess and alcohol fumes of a traditional alcohol and dry ice bath.
Next, fully charge the top pad of felt with alcohol
and evenly moisten — do not saturate — the bottom
pad. The alcohol on the bottom helps conduct heat
out of the chamber and hastens the formation of
the cloud. For this, you’ll want the highest concentration of alcohol you can find. Go to your local auto
supply store and purchase a bottle of Iso-Heat. This
product is used to remove water from fuel lines and,
as it turns out, is pure anhydrous isopropyl alcohol.
Now close up the chamber and wait. It will take a
while for all the dust in the air inside to settle out
and for the temperatures of the different parts of the
chamber to equalize. The show will start in about
20 minutes and it will go on for hours.
To see it, turn out the room lights. Then shine
a very bright column of light in from one side. A
bright flashlight will work, but the batteries soon
give out. I use the light from an LCD projector. The
placement of the light is absolutely critical. The
light must shine across the bottom of the chamber
where the droplets are forming, and you must position your head low and at a steep angle relative to
the light. Experiment a bit to find the positions that
best illuminate the chamber and provide the most
After the chamber has reached equilibrium, droplets will form very fast near the bottom and less so
as you move up. The top of the cloud will be about
4cm above the bottom. That’s where you want to
focus your attention, where the wispy telltale tracks
you are looking for will suddenly spring into being.
Their appearance, like tiny ghosts dashing through
a fog, is something out of Harry Potter.
If you measure their rate and directions, you can
learn a great deal. Consider adding a digital camera, shooting either through one side of the case,
or through a hole cut in the felt and wire screen on
the top. Take a steady march of images and keep
only those that happen to capture a track. Long,
straight, and slender tracks are most likely made
by muons. Thick, stubby tracks that start and stop
inside the chamber are created by alpha particles.
Their rate will let you set an upper limit on large
radioactive nuclei in your environment, like radon.
Once you’ve astonished yourself, make plans to
show your chamber to every person you possibly
can. No demonstration I know is better able to get
people excited about science than this one.
Dr. Shawn (Shawn Carlson, Ph. D.) is a former columnist for
Scientific American, and the only person ever to win a MacArthur Fellowship for science education. He is the creator of
the Society for Amateur Scientists ( sas.org), Super Science
Fair Support Center ( scifair.org), and Labrats ( labrats.org).