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[edit] The Cloud Chamber by the MARIACHIs

History|How to build one|Operation|How to record images|Related Information

[edit] History The cloud chamber is one of the most interesting and exciting instrument ever created to study particle physics. To date there is no other instrument that display tracks produced by elementary particles so well as cloud chambers, with the exception of bubble chambers. The cloud chamber was invented by Charles T.R. Wilson in 1912 to study cloud formation. His device, also known as the Expansion Cloud Chamber, had a chamber with gas saturated with water vapor. A sudden expansion of the chamber cools the gas, and cloud-drops are then formed instantly around the ions produced along the tracks of the particles. His device was widely used by many researchers in the field of nuclear and particle physics. One of the most striking discoveries made with a cloud chamber was the positive electron, the positron (or anti-electron), in 1932. Carl Anderson photograph shown on the left is the first evidence of this. His ingenious and simple experiment uses a cloud chamber in a region with magnetic field and a 6mm thick lead plate. In the image, the particle produced in the cosmic ray shower (coming from the bottom in the image), looses energy in the lead plate as seen by a smaller radius of curvature. An electron would have this exact trajectory as seen in a mirror. Anderson shared the 1936 nobel prize with Victor Hess and also discovered the existence of muons. The expansion cloud chamber has a supersaturated vapor for a short time, and hence required repeated pumping for its operation. A new cloud chamber was later introduced by Alexander Langsdorf in 1936: the diffusion cloud chamber. In a diffusion cloud chamber the layer of supersaturated vapor is created by cooling down alcohol vapor. This type of chamber become the favorite of many physicists by 1939 and continued to be the detector of choice to study particle physics until the mid 50's, early 60's, when the bubble chamber was introduced. The cloud chamber was used in many new accelerators of the time including the BNL Cosmotron. In 1948 Patrick M.S. Blackett won the nobel prize for his studies done with cloud chambers. His Nobel prize acceptance speech is very interesting and a must read for anybody interested in these devices. Today cloud chambers are mostly used for demonstrations and commercially available. The technology available today, however, permits to build a relatively inexpensive cloud chamber with magnetic field to explore elementary particle physics in the classroom. From the detection of electrons, muons and protons to electron-positron pair creation, a properly built cloud chamber is still a fascinating device and an invaluable educational tool.

How to build a cloud chamber with Magnetc Field

The diffusion cloud chamber operates in the principle that ionization triggers the condensation of supersaturated vapor. To create a layer of supersaturated vapor alcohol is frequently used (ethyl, methyl or isopropyl). In a container alcohol vapor produced at the top goes down to a colder region where it becomes supersaturated. While there is quite a bit of hocus-pocus in producing a thick layer of supersaturated vapor, the instructions given here are good to build and operate a chamber to visualize plenty of tracks ( 1 every 5-10 s) produced by cosmic rays interacting in the atmosphere.

The Materials List: 1 2.5 gallon fish tank (from pet shop) 1 4x6 500 Gauss Magnet (from Armstrong Magnetics Inc) 1 Anodized Aluminum Plate with drill holes placed in center of plate (or painted black) 4 Aluminum Heat Sinks 4 2 inch strips of Velcro 2 Strips of black felt cut into 3x7 strips 1 Roll of Duct Tape 1 Small Styrofoam cooler (no lid required) 1 Liter of Pure Ethyl Alcohol 10 lbs of Dry Ice 1 Light Source (Flashlight or Overhead Projector) Strips of weather stripping to seal chamber bottom

This cloud chamber has a permanent magnet (quite powerful actually) that allows for the measurement of the particle's momentum. To start building the chamber you will need to collect the materials listed on the left. The bottom plate that will make contact with dry ice is the part that requires some skills and tools. You will need an aluminum plate and heat sink to make good contact with dry ice in the container below. Aluminum plate can be obtained from several sources. We used a 8x14x1/4" aluminum plate. It is better to have it black for better track visualization. We have anodized ours in black. Anodization can be expensive if you are only making one cloud chamber. In this case you can paint the plate in black using Epoxy based paint. Beware that certain type of paint can be easily attacked by alcohol. Four heat sinks each 1" aluminum rod are attached to the plate and make contact with dry ice.

The magnet is a 4x6x1" magnet that provides 500 gauss at surface. It is a very strong magnet so be careful handling it. There is one in this design and it is secured in placed by two screws (small screws in the picture) that holds an aluminum bracket. At this point it is interesting to map the magnetic field. If you have a magnetometer you're lucky, or you can build an inexpensive magnetometer. Usually you get as much as 400 gauss at the region where you detect particles.

With the plate built, you can start to work on the fish tank. Alcohol is stored in two felt strips that are velcro'ed to the top, in an arrangement that we call the hammock. If you plan to operate the chamber for 3-4 hours only soaking the felt strips with alcohol will be enough. If you plan to operate for longer, you can make two holes in the glass (as shown in the picture) to feed more alcohol. Beware - drilling glass is not for the faint of heart! Add the weather strips to the bottom of the chamber - you will need this so that the internal environment does not come in contact with the outside environment.

Now for the dry ice container. We usually make a mix of dry ice and alcohol, at the bottom. So what you need is a reservoir that can contain this mixture. We use a shipping styrofoam box and fill the unecessary volume with scraps of styrofoam. The top is then covered by a thick plastic.

Operation

The first step is to prepare the cold mix. If your dry ice is a large block, you will need to chop it. Use first a screw drive to chop it into large chunks and then put them inside a plastic bag and hammer it. It is a good exercise. Mix the dry ice with alcohol (it could be any alcohol as long as it has low water content). This mix should reach -70C. The next step is to soak the felt strips (hammock) with pure alcohol.

You are ready to go. Imerse the bottom plate in the cold mix and put the fish tank on top. If you want, seal everything with duct tape. This provides an excelent seal and also avoids the top from moving. Place the source of light in one side of the chamber as shown in the picture and turn the room lights off. It works best if the environment is dark.

It will take approximately 10 to 15 min until you start seeing tracks. After 5 minutes, you will see mist forming. These are droplets of alcohol being formed due to low air temperature. After 10-15 minutes the bottom plate is cold enough to form a layer of approximately 1 cm of supersaturated vapor and tracks can be visualized. After about 20 min the bottom plate should reach a temperature of about -40C and the chamber is fully operational. The sensitive layer formed, assuming you are in a classroom, should be approximately 2 cm or less. It is easier to visualize the track at an angle where you almost look into the source of light (forward scattered light).

The best source of light is a strong flashlight or an old fashioned overhead projector. Illuminate only the bottom of the chamber. Conventional hand held flashlight also works but you may need more then one to fully illuminate the chamber.

...and voila - tracks. At this point you should see a plentitude of tracks.

How to record images

You will find out that it is pretty tricky to take pictures of tracks. Tracks will be visible only for 1s or so. The best way to record images is to use a digital video camera. You don't need professional equipment - any digital camera will do the job.

Set the camera on a tripod and adjust the angle so that you can see tracks. Adjust zoom or manual focus to look in an area that there is a lot of activity. Let the camera roll for a period of time that you think is adequate. Normally 20 minutes is enough to collect few very interesting tracks. Use your favorite video editor to capture frames from the video camera. And here's few examples:

• Overhead shot - Taken nearly straigth overhead shows one particle bending in the magnetic field
• Two tracks - This frame has two tracks also bending in the magnetic field.
• Big looper - a single track that almost makes an 180 degree turn.
• Heavy Ionization - Occasionaly very heavily ionization trails are seen. They are most likely protons.

Overhead shot

Two tracks

Big looper

Heavy ionization

If you want to get serious about measuring momentum, etc. you will need to put fiducial marks on the black plate.

Related Information

Cloud chambers provide an unique opportunity for you to detect particles of all types. A list of activities for the classroom was compiled during a recent QuarkNet 2006 workshop where we built these cloud chambers.