For the past few months I have been observing alpha tracks with my homemade expansion cloud chamber assembly, trying to replicate early achievements in particle physics using modern, low-cost materials. I've seen some interesting stuffs but I simply couldn't interpret the following phenomena.

enter image description here

These photos have similar scale, about 5 cm in height. Now I know these Y-shaped tracks are due to alphas "colliding" with nucleus of air atoms, but what intrigues me is the tiny but noticeable curvature (pointed by red arrows) of the recoil nucleus.

It is known that due to the mass of alpha particles, it takes incredibly high magnetic field strength to "bend" its path. Recoil nuclei (in this case most likely nitrogen or oxygen) are even more massive, so surely it shouldn't show curvature.

Nobel laureate P.M.S. Blackett published this finding in a 1923 paper (https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1923.0041) *note plate caption number 3. He was working on it then.

I couldn't find a follow-up on that. Was it experimental error, instrumental artefact, or something fundamental?

  • $\begingroup$ What does the energy loss curve vs energy look like? What energy loss mechanism prevails at lower alpha energy? $\endgroup$
    – Jon Custer
    Dec 23 '20 at 15:00
  • 1
    $\begingroup$ I think it is not a curvature but another collision. After the collision the alpha particles will slow down. The cross section is inversely proportional to the velocity because velocity (or momentum) inversely proportional to the wavelength (higher the wavelength (up to some limit) results in higher the cross section). So, alpha particles collide, slow down, cross section increases , collide again. $\endgroup$ Dec 23 '20 at 15:08

The straight lines are visible because of continuous small angle scatters of the alpha particle with the electrons of the medium .

The original forked scatter is on a nucleus, the apparent curve comes from the scattering of the low energy proton on another nucleus. If the consecutive scatter at the end of a low energy proton is in a different direction , the track struggles, but the thickness of the imaging hides it.

In the link you give there are bends in the other direction also .


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