Cloud Chambers

At the moment I am working on my Master’s thesis. As this is taking up a lot of my resources, I fail at generating more content for this blog. Geophysics related writing goes into the thesis right now. However, I’ve written a short piece on visualizing radioaktive decay in a diffusion cloud chamber. It’s not geo-content but interesting nonetheless. (The references are all to Wikipedia, just for further reading, so barely sources – if at all.)

The Cloud Chamber Cloud chamber

The chamber itself contains vapor, we use Isopropyl alcohol for this. The entire chamber is filled with this air-vapor mix, like a bathroom is filled with water vapor after a nice hot shower. On the bottom of this cloud chamber we have plate that is cooled down to -30?C (-22?F). The surface is very smooth to reduce the possibility of condensation.

Since we have heating at the top and cooling at the bottom we get a vertical temperature gradient. About 0,5cm (0,2in) above the cool plate the vapor isn’t quite ready to condensate and gets supersaturated. This basically means the partial pressure of the vapor is higher than its vapor pressureSupersaturation. So that layer above the plate contains more alcohol than it should. However, it won’t condensate because we’re missing an important part: condensation core (or nuclei)Condensation_nuclei. So basically something that will trigger the condensation.


Alpha decay
Alpha decay

And this is where radioactivity comes in handy. We have different kinds of radioactive decay, two of those can be seen in a cloud chamber. Namely it’s alphaAlpha_decay and beta decayBeta_decay. Those emit charged particles. Alpha decays emit a Helium nucleus, those have pretty high energy but short range. Beta decay either emit an electron or a positron. They’re usually less energetic but have much higher range. The radioactive gas, we blow into the cloud chamber is Radon. Radon is one of the gases that contribute to natural background radiationRadon.

Beta decay
Beta decay

However it’s not really easy to store, so we use a trick. Radioactive isotopes will decay in certain patterns from one isotope to another, this is called a decay chainDecay_chain. Since we want to utilize a certain decay (we’ll come back to this later), we can now see which solid isotope is suitable for our purpose. In our case it’s Thorium:

228Thorium -> 224Radium -> 220Radon -> 216Polonium -> 212Lead

We can see that at some point Thorium decays indirectly to Radon, our desired gas. We blow this into our cloud chamber and enjoy the radioactive magic. 220Radon does an alpha decay and emits a Helium nucleus, so it’s 216Polonium now. This special kind of Polonium is very unstable and has a half life of 0,15 seconds. When we stand 50 meters (165ft) apart, this is the time it would take for you to hear me yell somethingSpeed_of_sound. In our perception that is almost immediately. Polonium also emits a Helium nucleus, so this is the reason we see two big lines happen almost at once. Of course, if you’re very observant, you will see some of the lines clearly happening after one another. This goes a little deeper into radioactive decays. As radioactive decay happens randomly, the 0,15s half life just gives a mean timeHalf-lives, when half of the polonium will decay, the other half will probably take longer. (This is where statistics come into play and I’ll just leave it at that.)


Impact Ionization
Impact Ionization

This isn’t quite it, but up to this point we’re done with the radioactive decay. We understand that there are two alpha decays that happen almost immediately. Both emit a Helium nucleus, but why would this nucleus cause a glowing line in the supersaturated vapor? The supersaturated vapor is in a so-called metastable state. It would condensate if it had a condensation nucleus. This is where the high energy Helium nucleus comes in. On impact with other particles this high energy particle will ionize other particles around. In this process alcohol particles become charged and can therefore serve as a condensation nucleus. This ionization charges particles by kicking electrons out of the alcohol molecule. These electrons will then ionize other alcohol particles, like an avalanche. Hence, causing the trace of condensed vapor. The Helium nucleus itself would only have a maximum rangeRange_(particle_radiation) of 3-4 cm (1.6 ? 1.8 in) in free air but the trace is at least twice as long.

Bragg Curve for Alphas
Bragg Curve for Alphas Source

The traces itself are visible because the little droplets in the fog cause Mie scatteringMie_scattering of the light. This is the so-called Tyndall effectTyndall_effect that causes light to be radiated outwards of fog and clouds.

Tyndall effect in different media
Tyndall effect in different media
(Tea, colloidal silver, water)
CC-by-sa 3.0/de Pies und Braach

After every “glowing trace” we can see a black trace, which is due to the temporary absence of the supersaturated vapor after condensation.

Here’s an image of our chamber at DeSy on Wikipedia

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... is a geophysicist by heart. He works at the intersection of machine learning and geoscience. He is the founder of The Way of the Geophysicist and a deep learning enthusiast. Writing mostly about computational geoscience and interesting bits and pieces relevant to post-grad life.

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