A Simple Cloud Chamber
Arthur Kurlej, Jasmine Abdollahi, Andrea Pocar
Physics Department, University of Massachusetts, Amherst
What is it?
A cloud chamber is an air tight environment that contains supersaturated alcohol vapor, which
allows one to see the interaction of charged particles with the alcohol vapor. With such a
chamber, one may see the trails that charged particles (muons, electrons, or helium nuclei) leave
as they interact with alcohol molecules. These trails have distinctive characteristics, which allow
one to determine what sort of particle interaction was just witnessed.
What is needed?
A cloud chamber relies on a few basic principles, mainly, a supersaturated environment and a
sealed chamber. The goal of this write up is to showcase a simple and cheap method of
accomplishing this, while also providing resources to scale up and make a chamber that is more
impressive. Below is a schematic diagram of a cloud chamber:
The above setup can be easily accomplished with home supplies.
1) A clear plastic cup (taller than it is wide)
2) A piece of felt (thick felt works best!)
3) A thin flat metal plate (think a metal jar lid or baking sheet) larger than your plastic cup
4) Glue (make sure that alcohol will not dissolve it, something like Gorilla Glue works)
5) Weather stripping sealant/caulk cord/plasticine
6) A piece of Styrofoam larger than the metal plate
7) Dry ice
8) Some black construction paper
9) Isopropyl/Ethanol Alcohol (Be sure that it is 100% pure, and I also recommend using the
isopropyl variant)
10) A light source.
Optional) A radioactive source (something that undergoes alpha/beta decay will leave easily
noticed trails)
How to Build?
1) Take your felt and cut it so that it fits into the
bottom of your cup. Once this is accomplished, use
the glue to hold it in place as pictured. This felt will
be the reservoir for the alcohol that will ‘mist’
down.
2) Use your caulk/plasticine/weather sealant and
place it around the rim of the cup, making sure that
it is securely placed and covers both the inner and
outer edge of the cup. This will help provide an
airtight seal
3) Cut the black paper into a circle whose diameter
is slightly smaller than that of your plastic cup.
Place the paper circle onto the center of the metal
plate, and if you a radioactive source, place that
onto the center of the circle. (The source pictured to
the right is Americium-241, which nabbed from a
smoke detector). The paper will allow one to more
easily see the trails.
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4) Take your dry ice and place some ontop of your
Styrofoam container. Be sure that there will be
enough dry ice to adequately cover the entire
bottom of the metal plate, and be sure that the
Styrofoam container allows the metal plate to make
contact with most of the dry ice. Good thermal
contact is required in order to create the temperature
gradient required to have a supersaturated
environment.
5) Take your alcohol and thoroughly soak the felt,
up until the point that it cannot hold anymore. Pour
out any that was not absorbed by the felt. Then,
place the cup onto the metal plate, being sure that it
contains the paper circle within it. Press down onto
the sealant and be sure that it holds strongly to both
the metal plate and the cup itself.
6) Place the whole assembly on top of the dry ice,
turn down the lights, and shine your flashlight/light
source at a downward angle into the cup. Then,
watch and wait!
What you should see!
With the above assembly, almost immediately you should start to see the alcohol misting down
towards the bottom. Then, in less than five minutes, you should start to see little ‘spiderweb-like’
strands falling down. Congratulations! You just saw your first particle interaction!
There are a few different types of tracks that you may have witnessed.
-If you see a track that has gone straight and then sharply turns left or
right, you have witnessed a muon decay! The dashed lines are
neutrinos, which are not detected in the chamber.
-If you spotted three tracks meeting at a single point, then you
witnessed an incoming cosmic ray colliding with an atomic electron!
-If you see a very windy chaotic track, then you have seen ‘multiple’
scattering,’ which is the bouncing of a cosmic ray from one atom to the next!
-If you included a radioactive source within the chamber, then you will spot the above and also
see relatively straight lines of different thickness ‘shooting’ off from the source. The thicker lines
are the result of alpha decay, and the lighter lines are the result of beta decay (the difference in
thickness is due to the different energies of the decay particles, the thicker line results because
the alpha particle has higher energy and is therefore more ionizing).
What if I don’t see anything?
There are a few possible solutions if you don’t see anything. Usually, the biggest problem is
some sort of air leak (which you may see if you see CO2 within the cup). To fix this, you would
have to restart, and replace the seal. If your container is fairly sturdy, then pushing down upon
the container while it runs is also a good idea.
Another issue may be a lack of alcohol in the container. If thin felt was used, try and glue several
more pieces on or replace with a much thicker felt.
The source of light may also be shining in a bad orientation, so work on moving that around.
Generally, the action is just above the bottom of the chamber, so try to angle the light down, and
move it around and see if you can spot anything. Getting a brighter source is also an option.
If the all the above fails, try once more, and if resetting it did not work, a new, differently
shaped, container may have to be used. Try and use one that is a little taller or a little smaller.
Awesome! What else?
The above are instructions for a simple and fairly cheap set up, but it can be made to be much
more impressive. One can ramp up the quality of construction so it looks a lot more impressive,
one can replace the dry ice mechanism to one that works off of adiabatic expansion or peltier
coolers, include an electromagnetic field and see the trails bend in correspondence, or set up
some sort of a system to record and count events. Although it may seem devilishly simple, the
inventor of the cloud chamber (Charles Wilson) won a Nobel prize for it in 1927!
An example of a scaled up cloud chamber
Resources and Thanks
Suzie Sheehy’s video of a plastic cup chamber was the inspiration for ours!
http://www.youtube.com/watch?v=400xfGmSlqQ
http://suziesheehy.co.uk/
The below links provides a very thorough write up on a more elaborate setups
http://w4.lns.cornell.edu/~adf4/cloud.html
http://teachers.web.cern.ch/teachers/document/cloud-final.pdf
http://quarknet.fnal.gov/resources/cloudchamber.shtml
http://www.instructables.com/id/Make-a-Cloud-Chamber-using-Peltier-Coolers/
And general cloud chamber information
http://www.nobelprize.org/educational/physics/observing/cloud-1.html
http://www.cloudchambers.com/