Scattering Experiment

Geiger and Marsden's apparatus.
Alpha particles (from R) scatter off the foil (F). The microscope (M) is rotated around the cylindrical box (B) to count scattering at any angle

4. Geiger and Marsden

In 1907 Rutherford left McGill University in Canada and moved to the University of Manchester, where he began work with Hans Geiger.

They continued to investigate the scattering of alpha particles. The particles came from a small sample of radon-222, a radioactive gas produced when the element radium decays. These particles were directed through a vacuum and on to a foil which caused the scattering. The positions of the scattered particles were seen as scintillations, or small flashes of light, which were detected using a microscope that could be rotated around the foil.

The experiment was quite difficult. Observations could only be taken in a dark laboratory, and it took about half an hour for the observer's eyes to adjust to seeing the traces. Each observer could only count accurately for about a minute before they needed to swap over. Geiger later developed the 'Geiger counter', which could count these pulses automatically in normal light. It made similar experiments much easier!

The alpha particle scattering was usually only one or two degrees. In 1909 Geiger needed an experiment for a research student, Ernest Marsden. Rutherford suggested that Marsden look for alpha-particle scattering at large angles. He didn't think it very likely.

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