Particle track
Inside the trophy case cloud chamber when chilled and illuminated by an LCD projector. Although they do not show
up well in this photograph, the alcohol droplets and particle tracks are clearly visible to the naked eye.
remaining dry ice up in the towel to prevent it
from “smoking.” The alcohol will evaporate from the
warm sides of the jar and condense near the frigid lid.
Do this in a darkened room. Shine a bright flashlight into the jar from the side, and you’ll see
pinprick-sized droplets of alcohol coalescing near
the bottom. After a minute or two, when the dust
Muons fall from the
sky in all directions
as a steady spray of
subatomic machine
gun bullets that we
call “cosmic rays.”
Photograph by Shawn Carlson
inside the jar has settled out, you’ll also see something extraordinary. About once a minute, just
above where the droplets are forming, a ghostly
line will suddenly appear and then disintegrate as
falling rain of alcohol. These spectral emanations
are caused by ionizing particles of radiation passing
through your jar.
These thin trails of vapor form because the ions
that the passing particles leave in their wake attract
the electrically neutral alcohol molecules, just like
a balloon that has been electrically charged by rubbing on someone’s hair readily attracts small bits of
electrically neutral dander. The alcohol molecules
just above where the cloud appears are almost, but
not quite, cold enough to form droplets. But when
a passing particle lays down a trail of ions, those
ions can pull together enough alcohol molecules
to entice droplets to form. These droplets coalesce
along the track and essentially amplify its width
over a trillionfold to make the particle’s passage
plainly visible. For obvious reasons, a radiation
detector that uses tiny droplets to reveal its quarry
like this is called a “cloud chamber.”
While the cloud chamber in a jar couldn’t be
simpler to construct, it has three drawbacks. First,
it’s small, so you often have to wait a long time
before another cosmic ray will happen to pass
through at just the right spot. Second, the curved
glass makes the tracks hard to see. And finally, the
show only starts after any dust particles inside have
settled, and it stops as soon as the alcohol has all
condensed. That limits you to just a few minutes of
good viewing.
A far better chamber would operate with a large
enough volume for tracks to appear every few seconds, have flat sides for clear viewing, and would
contain a reservoir of alcohol large enough to keep
the show going for hours.
Make: 157
