Bohr Model
Quantum and Nuclear

Alpha particles as tools

Teaching Guidance for 14-16 PRACTICAL PHYISCS

When Geiger and Marsden carried out the gold foil experiment in 1909, a lot was already known about alpha particles. Although the rare back scattering was a surprise to Rutherford, he knew that the bullets he was firing were helium nuclei.

Rutherford had been working on alpha particles for a number of years. This was before the development of the cloud chamber and mainly relied on recording alpha particles with photographic plates. Between 1903 and 1908, he had:

  • deflected alpha particles in electric and magnetic fields and determined their charge to mass ratio (half the value for a hydrogen ion). This meant that they were probably either He2+or singly ionised hydrogen molecules; Rutherford favoured the former.
  • with Frederick Soddy, published a paper in which they estimated the mass, energy and speed of alpha particles.
  • noticed small deflections of alpha particles by air and mica (by firing them through the target at a photographic plate).
  • tried to count and collect alpha particles to measure their charge; he later measured their charge (as +2e).

It was whilst trying to get a reliable counter that he and Geiger noticed the amount of scattering. At first, the scattering was a frustration. But once Geiger had noticed some large angles, they turned it into the famous investigation that began in 1908.

In 1909, Rutherford and Royds collected the gas that was formed when alpha particles were trapped in a tube and showed that it was helium. So they were now sure that alpha particles were doubly-ionised helium atoms, He2+.

However, because Rutherford still thought of atoms as plum puddings, he was still astonished when some of them were back scattered in the Geiger and Marsden experiment. It was sometime in 1910 that Rutherford put forward his idea of a nuclear atom and so the alpha particle itself could be referred to publicly as a helium nucleus.

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