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Cloud Chamber

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The trajectories of individual charged particles leave behind cloudy trails as they ionize the cooled, supersaturated air-alcohol vapor inside this diffusion cloud chamber. Alpha particles from the radioactive decay of an inserted 2% thorium alloy rod form dense condensation trails nearby. Farther from the rod (beyond 10 cm or so) various types of charged particles leave evidence of their activity, including alpha particles (dense tracks, perhaps from the decay of radon isotopes), muons and energetic electrons (faint, seemingly straight tracks), and relatively low-energy electrons and beta particles (faint, tangled tracks).

This movie shows a basic, qualitative demonstration of the presence of subatomic particles; techniques to more accurately identify and measure individual subatomic particles are possible with this apparatus.

The cloud chamber was first developed by C.T.R. Wilson around the turn of the 20th century to study optical phenomena associated with mist and clouds (he received the Nobel Prize in 1927). When charged particles ionize a supersaturated vapor, a trail of ions is left in the path of the particles. The ions act as condensation nuclei for the alcohol to condense on, and a thin line of fine droplets is formed in the path of each particle.

The range of the alpha particles from the thorium source is about 4 cm. As the alpha traverses its path, it slows down gradually and becomes more heavily ionizing by virtue of the fact that it spends more time in the vicinity of air molecules in its path. Evidence for this can be seen by observing that the tracks become denser with increasing distance from the source.

This particular diffusion cloud chamber was built by Supersaturated Environments of Madison, WI (http://www.cloudchambers.com). Dry ice pellets under the chamber floor help to create a steep temperature gradient from top to bottom. Felt strips along the top of the chamber are soaked with 95% ethanol. The viewing area is 51 square centimeters, with inscribed "+" marks every 10 centimeters. The special slide inserted into the Kodak Ektagraphic III projector saves the viewing audience from unnecessary glare.

REFERENCES

* W. Gentner "An Atlas of Typical Expansion Chamber Photographs" Pergamon Press, 1954.

* E.W. Cowan, "Continuously Sensitive Diffusion Cloud Chambers" Review of Scientific Instruments 21, 991 (1950)

* R. Cases, E. Ross, J. Zuniga "Measuring Radon Concentration in Air Using a Diffusion Cloud Chamber" Am. J. Phys. 79, 903 (2011)

For some more details on our setup see:
http://sciencedemonstrations.fas.harvard.edu/presentations/cloud-chamber

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