Watching a video of a cloud chamber on wikipedia (http://en.wikipedia.org/wiki/File:Cloud_chamber.ogg), I cannot help noticing the large collisions that take place at 00:12 and 00:24.

What are they? Alpha particles? They are huge compared to the small ones (i guess they are electrons)

  • $\begingroup$ Without a scale on the image and some info about the ionizing source it's very hard to tell. The caption on the associated image from en.wikipedia.org/wiki/Cloud_chamber suggest that they are alpha particles, but it is not clear to me why the author believes that. It the ionizing source is cosmic rays then the most common tracks may be muons (not electrons), in which case the heavy tracks could easily be protons. $\endgroup$ May 13, 2012 at 21:51
  • $\begingroup$ The main reason I'm not convinced that they are alphas is that they appear in the middle of the detector, and while cosmic rays can do that they need a heavy target, where as the many protons in the alcohol can easily be scattered by the neutron detritus of a cosmic shower. $\endgroup$ May 13, 2012 at 21:57

2 Answers 2


I tutor a cloud chamber workshop at CERN weekly, so I have some regular experience with recognising cloud chamber tracks.

There are only four particles (plus their antiparticles) we can observe in a cloud chamber:

  • $\alpha$ (He nucleus),
  • p (proton),
  • $\mu$ (muon),
  • $\beta$ (electron).

All other particles are either uncharged (and hence don't ionise the cloud) or decay too fast to survive at least a few millimeters (the minimal amount to be able to see them in a cloud chamber).

The width of a track depends on the ionising power of a particle, which in turn directly depends on its charge and its mass. From the above four particles, the $\alpha$ particle has charge +2 while the others have charge $\pm 1$. Their masses are roughly $m_\alpha = 4$GeV, $m_p=1$GeV, $m_\mu=0.1$GeV, and $m_e=0.0005$GeV. From this we estimate an $\alpha$ particle to be 8 times as ionising as a proton (with the effect of the double charge included), which in turn is 10 times as ionising as a muon, which finally is 200 times as ionising as an electron. This is of course merely a rule of thumb while the complete calculation is a bit more intricate. But the main result is correct:

The thick tracks are $\alpha$ particles or protons, while the thin tracks are muons or electrons.

But how do we distinguish between an $\alpha$ particle and a proton? Well, as you maybe remember from your high school nuclear physics class, $\alpha$ radiation can be stopped with a thin sheet of paper. In other words, there is no way that an external $\alpha$ particle could penetrate the casing of a cloud chamber. Hence, $\alpha$ particles that are observed in a cloud chamber are created inside. This can be due to an $\alpha$ source which has been positioned inside, but can also be due to natural background radiation: air contains a certain fraction of Radon, which is a natural $\alpha$ source. In our cloud chambers at CERN (which have no internal $\alpha$ source, so they only show the effect of Radon decay) this accounts to more or less 3 $\alpha$ particle tracks per minute (but the Radon concentration varies geographically, with the weather, and has some other factors, so it could be much more or less). Protons on the other hand cannot be created from radioactive decay, so they come from cosmic radiation.

Why does this matter? Because it is easy to distinguish radioactivity from cosmic rays based on their energy: while particles generated from radioactive decay have kinetic energies in the range of a few MeV's, cosmic rays have kinetic energies in the range of TeV - EeV (yup that's Exa-electronvolt, or $10^{18}$eV). This means that $\alpha$ particles, due to their huge mass relative to their kinetic energy, have very low penetration potential (that's why they are stopped by a sheet of paper) and can travel only a few centimetres in air. Protons are of much higher energy and can travel through the cloud unimpededly, and will hence form long straight thick tracks. Even though the length of a track also depends on the angle at which the particle crosses the cloud (implying that short thick tracks could be protons at a steep angle), it is known that very few protons are generated in a cosmic air shower (in one year of weekly tutoring workshops of 3 hours each, I have only seen 6 protons up to today). Furthermore, a last discriminating parameter is the particle's speed: cosmic particles move at speeds close to the speed of light, while (radioactive) $\alpha$ particles are so low in energy that they have speeds in the order of a few centimeters per second.

To conclude, the tracks from the video you refer to are thick, relatively short, and slow, so they are undoubtedly $\alpha$ particles.

Ps: the same reasoning can be applied to distinguish between muons and electrons. Muons can only be cosmic, while most observed electrons will be radioactive. Even though comparing the speeds won't be easy (even radioactive electrons move too fast for the eye to be called slow), we can make the difference based on the form of the track: long and straight implies high energy and hence a muon, curved and full of cusps implies lots of interaction so low energy, and hence an electron.

  • $\begingroup$ nice answer which brings a few questions: if the particles lose their power after a few centimeters in the chamber, how did they even got there ? coming from cosmic rays they would have to go through the sky first, then building walls etc. so why are they stopped after a few centimeters in the chamber? I understand for those made inside the chamber by radon $\endgroup$
    – Thomas
    Jan 14, 2017 at 20:46
  • $\begingroup$ other question : where do the electrons come from ? and the muons ? from the sun rays ? $\endgroup$
    – Thomas
    Jan 14, 2017 at 20:47

One can deduce the mass from the ionization range of the particles that left the trace in the cloud chamber. I will only copy a bit, but if you are interested read the article

. Since the mass of the proton or alpha particle is much greater than that of the electron, there will be no significant deviation from the radiation's incident path and very little kinetic energy will be lost in each collision. As such, it will take many successive collisions for such heavy ionising radiation to come to a halt within the stopping medium or material. Maximum energy loss will take place in a head on collision with an electron.

In this particular exposure there are faint struggling tracks making large scatters, and they can classified as electrons, there are straight tracks with minimum ionization ( same as electrons) which must be muons, and thick tracks with high ionization. Particularly the ones you point out are good candidates for an alpha since they have such high ionization are straight and stop.

It may be that they have placed a radiation source close by, one needs more information of the exposure. It is clear in the photo in the main article that they have a radiation source there.

The bubble chamber, with a magnetic field allowed real particle identification by the ionization of the tracks per cm and the curvature in the magnetic field which gave the momentum, thus complete interactions could be identified with the particle content known for large accumulated statistics. This has the disadvantage of needing a lot of scanning of pictures by eye. The advance of electronics and material science allowed the identification by ionization to be carried out digitally and with more accuracy for high energies.

The method of particle identification by ionization digitally is till in use in time projection chambers , TPCs.


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