Detecting Cosmic Rays in a Cloud Chamber




About: I am a physics student at Wheaton College who likes building interesting physics demos and other stuff that I think is awesome!
It is kind of crazy to think that we are being bombarded at all times by tiny particles moving close to the speed of light. If you want to see  evidence of these randomly occurring cosmic rays check out the video. If you still don't buy it or want to see the vapor trails for yourself (like I did) then you should continue reading this instructable.

Step 1: Materials

Here is what I used to build my dry-ice cooled cloud chamber:

- Insulation foam (I bought some that was conveniently cut to fit between the framing boards on a house which proved a perfect width)
- Fish tank 
- Metal plate
- Black Modeling clay
- Black Cardstock
- Black Electrical Tape
- Glue (to glue the foam pieces together)
- Sand paper (to smooth the foam)
- Black Foam Poster board (this was used to create a cover for the fish tank to make the pictures and video come out nicer)
- Flashlight or old slide projector (basically you need a solid beam of light to see the cosmic rays)
- Dry-Ice (Used to cool the base of the cloud chamber)
- 91% or higher Isopropyl alcohol (70% does not work!!!)
- Water (this was poured into the clay channel on the base to create an air tight seal)

- Razor Blade
- Paint Scraper (I used it to help cut the foam)
- Yard Stick
- Sharpie
- Hot Glue (used to attach the felt to the top of the fish tank)
- Magnets (used to attach the felt to the top of the fish tank)

Step 2: Constructing the Insulated Box (for the Dry-Ice)

Essentially you need to create a insulated bo that will be filled with dry-ice. The box  needs to be big enough so that the metal base plate can sit in it snugly. I used the metal plate a a guide to cut the box out of pieces of insulation foam. I then sanded the foam down and glued everything together. When I cut the foam I would first use the razor blade and then once a small scratch had been made in the foam I would punch all the way through with the paint scraper. I am pretty sure that there was a better way to cut the foam but with the sand paper I wasn't too concerned about how it looked initially. 

Step 3: Preparing the Metal Base Plate

The metal base plate sits on top of the dry ice to facilitate a temperature transfer. One thing that is of primary importance is that the metal plate needs to be able to maintain an airtight seal with the fish tank. This maintains a high concentration of alcohol inside the cloud chamber and allows for the formation of a super saturated alcohol vapor.  In order to do this I decided that I would fill a small groove around the edge of the fish tank with water or excess alcohol. In order to create the groove I used modeling clay and just molded two tracks of clay onto the metal plate. Then to make the white vapor trails contrast with the bottom of the could chamber I attached black card stock to the bottom of the baseplate. This made the vapor trails a lot easier to see.

Step 4: Modifying the Fishtank

Essentially all that I did to the fish tank was attach a piece of felt to the bottom (which turns out to be the top of the cloud chamber). At first I just used hot glue to attach the felt to the top of the fish tank. However as you can see from the picture It sagged down when alcohol was added to the felt. To fix this I used some sets of magnets that I had lying around to stick the felt to the top of the fish tank much more securely. The felt is ultimately just used to absorb alcohol and a sponge or other kind or absorbant fabric could easily be substituted in place of felt. 

Step 5: Putting It All Together!

Now the foam box for dry ice, the metal baseplate, and the fish tank are all ready for action! 

The first step is to put dry ice into the foam box and place the metal base plate on top of the dry ice. The metal plate will probably let out a screeching sound as the metal cools but this should subside after a while.

Then pour some alcohol onto the felt until the felt is pretty well soaked in at least 91% isopropyl alcohol and place the fish tank ontop of the metal baseplate.

Then pour some extra water or alcohol into the clay channel that you made in the baseplate to ensure an airtight seal.

Now darken the room and illuminate the cloud chamber with your light source. I covered all but one of the fish tank's faces with black poster board to make it easier for the cosmic rays to be filmed but that is not necessary. Almost immediately you should see a cloud of alcohol vapor start to rain down and after about 10 min a super saturated vapor of alcohol should form. Now you should be able to observe some vapor trails towards the bottom of the cloud chamber. 

Basically the alcohol vapor is so saturated that when a cosmic ray passes through the chamber it ionizes the molecules in the air and the alcohol vapor condenses around these trails of ionized molecules. This is how the cloud chamber allows us to "see" particles that are moving close to the speed of light. To start learning more about why this works check out the wikipedia link.



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22 Discussions


5 years ago on Introduction

Did you have any issue actually seeing the ionization trails with a camera? OR would it be difficult to measure the velocity of these particles and things like that with a video camera?


6 years ago on Introduction

Hey Guys,
I don't want to put a damper on the fun, but isn't this a bomb looking for an ignition source?

2 replies

6 years ago on Introduction

Very nice! This is a terrific project, and quite well described. There are some interesting ways you can extend this and do some quantitative study of cosmic rays.

1) Add an attenuator. By putting sheets of steel or lead above the aquarium (you'll want a frame to support them!) you can limit the cosmic rays passing through to just those above some energy. Varying the thickness allows you to estimate the energy spectrum.

2) Look for antimatter. Cosmic rays are electrically charged: muons (like electrons) have a negative charge, while antimuons (and positrons) have a positive charge. If you put a magnetic field across your chamber, from front to back, particle of each sign will be curved in opposite directions, instead of travelling straight down. You can make a uniform field with a pair of Helmholtz coils, and you'll still be able to see the chamber. Knowing the orientation of the field you can figure out which particles curve which way, and you can tell the electrons (which will curl up in tight spirals) from muons or pions.

2) Add a trigger system. The way you have this built, you can only sit and watch it, and see the particle tracks appear and drift away. With additional equipment, you can set it up to take a picture each time (and only when) a cosmic ray passes through. A couple of sheets of plastic scintillator with photodetectors glued on to them for readout, can be fed into a simple coincidence circuit (essentially just an AND gate) to trigger a digital camera.

8 replies

Reply 6 years ago on Introduction

What you offer is true in a theoretical sense but totally impractical for an amateur, consider this before using up time and money.

1. There will not be that many traces in a rig like this. You would have to know the energy level of the particles being stopped. Since cosmic rays are better stopped by high hydrogen plastics like polyproylene rather than metals you might be able to slightly vary the number. There are secondary radiation particles created which complicates the issue. For example, thin pieces of aluminum actually INCREASE the amount of radiation through the creation of secondary radiation particles. Calibration for useful results would be problematic since the material would have to be: 1. one with precisely known absorption charateristics, 2. available, 3. and affordable.

2. The magnets usually used are very big, far larger than anything an amateur would have enough power to run even if they could build it. Small setups are far beyond the technical cpability of the amateur, the ones used on space exploration vehicles are very expensive and complex to build involving sophisticated design and materials.

"A couple of sheets of plastic scintillator with photodetectors glued on to them for readout, can be fed into a simple coincidence circuit (essentially just an AND gate) to trigger a digital camera."

3. If you can build a cheap scintillator with a simple circuit and some 'scintillator' material you have laying around your lab, you have a very wealthy future ahead of you. The ones I have heard of are quite expensive and use somewhat exotic materials, some of the crystals used take months to grow.

It's a fun project, just build it and watch it.


Reply 6 years ago on Introduction

I don't disagree completely with anything you've said, but I don't think the barriers are quite as high as you think.

Taking the last point first, you are absolutely right that it's not trivial, and would cost some money. Building a remote camera trigger is a project which has been shown multiple times in MAKE magazine as well as in various Instructables (you can do it mechanically with a solenoid, rather than wiring a circuit into the camera). Plastic scintillator sheets are pricey -- I found a 6" square for $60. Of course, the price and complexity is why I listed it last.

As for stopping and attenuation, lead or steel are excellent materials. Near sea level, there are essentially no primary cosmics at all; they've all interacted in the upper atmosphere, and what we see down here are the shower secondaries, almost entirely muons (the charged pions decay in flight). Those muons typically have momenta of a few hundred MeV/c (since the spectrum falls off like E^-3), and lose energy entirely through ionization (dE/dx). Muon-nucleus interactions (part of my research focus) are miniscule by comparison: you won't see multiple secondaries coming out of a metal plate unless you get really lucky!

The few-hundred MeV/c momentum scale for most sea-level cosmics also means that you don't need a really strong magnet if all you want to do is separate positive from negative charge. A convenient rule is that a 300 MeV/c particle has a cyclotron radius of 1 m in a 1 T magnetic field; the radius of curvature scales linearly with momentum and inversely with field strength.

What you care about to identify positive from negative is the saggitta, or equivalently the radial divergence with path length (i.e., if a pair of parallel particles hitting the top of your chamber at the same place, how far apart would they be at the bottom?). Let h be the height of the chamber, and R be the "typical" cyclotron radius (computed as above). In traversing the chamber, the particle will curve through an angle Q = cos-1(h/R), and at the bottom will be shifted by L = R(1 - sin Q). Some trig will let you write L directly as L = R - sqrt(R2 - h2).

Using h=30 cm (a typical aquarium) and a "typical" 300 MeV/c muon, you can immediately extract L for various R: at R=1 m (1 T magnet, hard to make safely), L = 4.6 cm, which is easily distinguishable by eye. At R=10 m (1 kG magnet, still quite strong), L = 4.5 mm, small but definitely photographable. At R=100 m (100 G magnet, about the level of an elementary school wrapped nail), L = 0.45 mm, which is probably about the edge of what you can see without a good calibration reference for the camera.

So I would claim that with a few hundred gauss electromagnet, which can be built at home by the same sorts of folks who build Tesla coils :-), you should be able to get charge separation in the author's cloud chamber.

Finally, you started by talking about rates. For secondary cosmic rays, the rate at 1 GeV is 104 m-2 s-1, or 1 per second per square centimeter. That rate is even higher for lower energies (due to the E-3 scaling). With the author's aquarium, the area is about 1500 cm2, so there will be a constant flux of tracks without attenuation. Rate is just not an issue.


Reply 6 years ago on Introduction

Because what I've described is a simple, crude method to perform some basic measurements of cosmic ray properties.

The experiment I worked on for the first ten years of my career cost $150 million dollars. It had approximately 200,000 channels of high-rate, digitized electronic readout spanning five different kinds of particle detector systems. It had 10 micron position resolution for charged particles, about 10 MeV energy resolution (for 1 GeV photons). It had a 1.5 T superconducting magnet system to provide 1 MeV/c momentum resolution for charged particles; the flux return of the magnet system was instrumented to allow tracking of muons out of the detector apparatus.

If you want high precision, high rate, cutting edge research equipment, which is one of a kind (and so not amenable to mass-production economy), then you are going to pay for it.

If you want to do decent citizen science, and learn about how to design, build, operate, and troubleshoot custom apparatus, you can do that for much lower cost.


Reply 6 years ago on Introduction

I am glad you have such an enthusiasm for your work. A lot of people don't associate passion and science which simply means they don't know any scientists, nor do they grasp the quality of mind for the pursuit of scientific knowledge.
Passion is good in any art/science/pursuit that requires years of dedication.

I remember reading an article in the 'Amateur Scientist' in Scientific Amercian from over 4 decades ago when I was taking physics in college. They had plans for an apparatus designed as a small particle accelerator that someone had built but obviously was unfamiliar with how much radiation it would generate. The magazine was beseiged by letters written by scientists in the field pleading to print a desperate warning NOT to build it as the specifications for the shielding were totally inadequate for the radiation levels it would create.

Thanks for supporting SAFE citizen science.

Thanks for your time.


Reply 6 years ago on Introduction

You're most welcome, and thank you for your questions and comments!

Citizen science is critically important, if we, as a society, are to have a literate and informed populace. There is a tremendous amount which can be done without special resources, funding, or construction, but those areas don't always engender the kind of passion that building something, and discovering something new (to you!) can.

Thanks for the great suggestions! I will have to look into setting up some Helmholtz coils inside the chamber and try to see if I can get a magnetic field strong enough to observe some curvature in the particles paths.

Sorry about the screwed up link. You should be able to set up the coils outside the aquarium and still get a decent field. I suspect that the field will not be uniform enough to do a quantitative momentum measurement (i.e., fit the track to a circle), but you should definitely see +ve vs. -ve curvature. That's how the positron was discovered, after all ;-)


6 years ago on Introduction

Is dry ice for keeping the vapour saturated? What about one of those chepo humidifiers (piezo i suppose). Would that do the job?


6 years ago on Step 3

Is that steel? Diamond plate? Where did you get it?


6 years ago on Introduction

Interesting 'ible! I was aware that the process was simple but wasn't aware just how simple. Thanks for showing us how to do it with ordinary household items. I should like to make two suggestions: First, there's no need to pre-cut the insulating foam with a knife. Just use a file to sharpen one side edge of an ordinary paint scraper and run the new blade through the foam against a straight edge or piece of wood. It will do all the cutting for you and give you straighter results. Second, Don't breathe too much of the isopropyl alcohol fumes. The stuff can impair your motor coordination and do other nasty things to you.


6 years ago on Introduction

I've been following Instructables for several years, and this may be the COOLEST thing that I have ever seen on here. I'm simply astounded. I've got to find a science teacher to share this with!