Building a Particle Detector

The Earth is continuously showered in cosmic rays. Their collisions in the upper atmosphere produces elementary particles that rain down upon us in huge numbers. There’s no need to worry though, since these elementary particles pass straight through you with barely any effect. However, it is possible to build a simple particle detector that will allow us to see their movements. Specifically, this article will show you how to make a Continuously Sensitive Diffusion Cloud Chamber.

This video shows what you’ll be able to see. Each white line that forms is the result of a particle passing through the detector.

There are quite a few different designs available on the Internet (discussed later); but I think the design shown here is one of the simplest, and gives quick, reliable results. However, there’s no getting away from the fact that you will need two mildly exotic materials to build it:

  • Isopropyl Alcohol: This is a very pure form of alcohol (I used 99% pure). This is easy to order from the Internet and is inexpensive.
  • Dry-Ice: Frozen carbon dioxide. How easy it is to find will depend on where you are in the world. Apparently in the US you can just buy some in a local store. In the UK, I had to order it online from a specialist supplier and have it delivered by courier. There are minimum order sizes (2.5KG from Chillistick) so this will be your biggest expense. I’ve seen suggestions to contact local fishmongers, butchers or ice-cream makers since you don’t need very much.

Everything else you will either have lying around your home, or can be easily found for very little cost.



Dry ice does not last very long because it literally evaporates into thin air, so I recommend getting everything together and building your detector before buying the dry-ice. You will need:

  • A plastic cup: medium to large in size and as transparent as possible.
  • A foil tray: the disposable kind (see slideshow further down) that will act as a base.
  • Black electrical tape: that you will cover the foil with.
  • A small piece of felt: or similar absorbent material.
  • Something to hold the felt in place on the base of the cup: I used a magnet and metal washer.
  • Plasticine or blue-tack: to seal the cup to the foil base.
  • Polystyrene block: ideally a solid block that you can carve a perfectly sized cavity into.

The overall arrangement is shown in the diagram below:

Construction of the cloud chamber particle detector.

Construction of the cloud chamber particle detector.

The polystyrene block is optional.  You could just spread out some dry-ice on a tray (protect any surface that it is resting on), and then place the cloud chamber directly on top. However, the aim is to cut a circle out of the foil, and then cut a cavity out of the polystyrene block of exactly the same size. That way there will be a snug fit between the two, restricting the leak of carbon dioxide, and reducing the build-up of condensation on the sides of the cup.

Some sites suggest using black card as part of the base; but when I tried it I had poor results. I think it was a combination of the card insulating against the cold and absorbing some of the alcohol. Electrical tape worked much better because it is thinner and water-proof.

The completed cloud chamber is shown below (some black card was added to the top of the polystyrene block, but it is purely cosmetic):

The completed cloud chamber.

The completed cloud chamber.


What are you Seeing?

“Particles that bombard the Earth from anywhere beyond its atmosphere are known as cosmic rays” (Cosmicopia).

These particles will have come from violent events out in space, mostly involving stars – such as solar flares and supernovae. Although these rays haven’t (yet) gifted anyone with super-powers; they do pose a potential hazard to astronauts venturing away from Earth, and at the very least have been known to cause them to see flashes of light.

The cosmic rays that reach Earth mostly collide with atoms in our atmosphere, causing both to explode into simpler combinations of particles. The particles that show up in our detector – and pretty much the only ones that make it to the Earth’s surface – are called Muons. The inappropriately titled Muon Basics page (there’s nothing basic about it) explains how they’re produced. The page also explains that having them reach the Earth’s surface is evidence for the theory of relativity – they normally wouldn’t survive long enough to make the journey, but travel so fast that time slows down (relatively speaking).

Muons have the same negative charge as an electron, but are 207 times heavier. As muons pass through atoms in the air they can easily dislodge the lighter electrons, leaving a trail of ionised (electrically charged) atoms in their wake. Normally this wouldn’t have any noticeable effect; but inside our detector – where the air is saturated with alcohol droplets just looking for something to condense onto – they cause trails to appear.

Alcohol droplets in the air (left); charged particle passing through leaving ionised trail (centre); droplets attract to ionised trail and condense (right).

Alcohol droplets in the air (left); charged particle passing through leaving ionised trail (centre); droplets attract to ionised trail and condense (right).

Of course, you might detect things other than muons. The Symmetry Magazine website has a nice breakdown of the different trails that you might see, and what causes them. It also has an alternative design for a cloud chamber, if you have a spare fish tank.


Sources of Radiation

What if you don’t want to passively wait for particles to show up, and want to command their appearance instead? In that case, you’re going to need some sources of radiation.

Radioactive materials are, understandably, a little difficult to come by. Depending on your location you might be able to buy some from the Internet (e.g. search for “radiation needle sources”). Some sites also suggest old clocks as a potential source, since they used to use radium to make the dials glow in the dark. However, some research identified two easily obtainable, low-cost sources:

  • Thoriated Tungsten Electrodes: Used in welding. It is the Thorium that is radioactive.
  • Ionisation Smoke Alarms: There are two types of smoke alarm – optical and ionising. The ionising type use a radioactive source (you will need to take the smoke alarm apart) – most often Americium 241.

Both sources emit alpha particles – which is the most destructive but also the most easily blocked (a sheet of paper is enough). The general consensus is that if you wear gloves then the only real threat is from ingestion (deliberate or accidental). Only small percentages of the material might actually lodge itself within your body (see hazards); but some residue would essentially sit there for the rest of your life, firing out cancer-causing radiation.

I tried both. They weren’t used under ideal conditions because I was experimenting with different setups, but I didn’t see anything spectacular. I was also paranoid about contaminating the area that I was working in, and potential risks (no matter how small) to my family. I was probably overreacting. In all honesty though, I don’t think they’re necessary – I was quite happy watching the end result of stars exploding millions of miles away.


Other Designs

I think that the design shown on this page is fairly solid. I created two cloud chambers from different sets of materials, on two consecutive nights, and both worked perfectly. However, you might want to have a look at some of the other designs available. There’s lots of choice in the container – from fish tanks (although larger ones seem more temperamental) down to petri dishes:

Because the dry-ice can be the most difficult item to source, I looked at a few designs that didn’t need it. The first was a design from Scientific American – billed as the world’s simplest particle detector – that uses an aerosol can to cool the base. When I tried it no vapour appeared inside the chamber. However, I was using card in the base – which doesn’t perform well – and it was from their suggestion that I tried electrical tape. There is also a whole branch of designs that use Peltier coolers (electrical heat pumps), ranging from relatively simple designs like the Instructable by nothinglabs, to some significant feats of engineering:

Another set of designs don’t use a cooling source at all. These are Expansion cloud chambers, and they use rapidly changing pressure to produce the mist of alcohol. I tried a few designs using jars – similar to the caviar jar expansion cloud chamber from Teralab (although my jars were mostly from pesto and curries). Again, I couldn’t produce a mist; but these designs rely on air-tight seals and my construction was probably deficient.

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