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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)

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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. –  dmckee May 13 '12 at 21:51
    
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. –  dmckee May 13 '12 at 21:57
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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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