Cloud Chamber Help

  • #1
Yesterday I decided to carry out a cloud chamber experiment for a school assessment. I used "100%" isopropanol and around 2.5 kg of dry ice. After a wait of approximately 30 minutes, the particle tracks began to appear. I placed a weak source of Americium-241 sourced from a smoke detector (1 μCi) in the chamber, expecting to see some thick alpha particle tracks, but instead got thin and short range tracks. What could these possibly be? Are they secondary particles produced by the small amount of gamma emission from Americium-241? The reason I am having so much difficulty identifying these is that the electrons or positrons have somewhat 'curved' or 'curly' tracks and are much longer. I have attached a video of the tracks, you may need to look closely as they are very small. Any help identifying these would be greatly appreciated. Thanks.



 

Answers and Replies

  • #2
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I would bet, that it is more likely, that you observed secondary cosmic radiation, mainly muons, rather than Am decays. However, I have to admit, I couldn't see very well from where to where the rays have gone.
Wikipedia says we have a flux of ##100 \;m^{-2}s^{-1}## muons on the ground and that they are hard to shield, due to their high energies. Whereas ##^{241}##Am as used in smoke detectors has a half-life of 432 years. So I put my money on the muons.
 
  • #3
I would bet, that it is more likely, that you observed secondary cosmic radiation, mainly muons, rather than Am decays. However, I have to admit, I couldn't see very well from where to where the rays have gone.
Wikipedia says we have a flux of ##100 \;m^{-2}s^{-1}## muons on the ground and that they are hard to shield, due to their high energies. Whereas ##^{241}##Am as used in smoke detectors has a half-life of 432 years. So I put my money on the muons.
Hi, thanks for the reply. I had initially thought that the particles were muons, however the tracks they leave are much longer and straighter. About twenty minutes in to the experiment, I observed muons and they were very different to these unknown particles which seem to have originated from the small sample of Americium. Here is an image of what a muon looks like:
ccmuon.jpg
 
  • #4
What you see with an Americium button is mainly beta decay, because the alpha particles are being shielded for safety. You can see the effect of shielding by using a Thorium doped welding rod where 1/2 of the length of rod is also encased in typical heat-shrink tubing. From the uncovered end you'll see alphas and betas but from the end covered with heat-shrink you'll only see beta particles. The heat shrink shields the alphas in spite of how thin it is. In most smoke detectors, the Americium cup has a thick back layer as a base which shields alphas and betas while also using a thin front cover to filter out the alphas, so you have a nice source of low energy betas for smoke detection.
 
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