Alpha-Particle Scattering

Objectives

  • To understand the alpha-particle scattering experiment
  • To appreciate the key points of this experiment which made it a success

You will need to have a solid understanding of the significance of the alpha-particle scattering experiment. You will need to know what, and the reasons why, the experiment gave light to a new model of the atom. In addition you will need to be able to explain the experiment itself as well the key features of it.

The Use of Alpha Particles

  • Alpha particles  ^{4}_{2} \alpha  are very small (they consist of two protons and two neutrons) and yet have enough mass (4 atomic units)  o be considered to have enough momentum to bombard the gold leaf with.
  • Alpha particles are produced naturally  by radioactive nuclides that are alpha emitters and so a steady supply was easily obtainable – they had been researched for the 10 years prior to this research.
  • Radioactive isotopes emit radiation in all directions. In order to ensure the alpha particles were directed in a chosen direction (I.e perpendicularly to the surface of the gold leaf), a collimator was used. A collimator is a device that absorbs radiation in all but a chosen direction (they are usually made from lead and so have the ability to absorb, at the very least, alpha and beta particles.

The Foil

  • A single atom would have been ideal to have been the target and therefore to have determined the structure of. However, if there were only one atom it would have been extremely difficult to have targeted it with the alpha particles. A single atom is not visible to the human eye and therefore it would have beenimpossible to just hotness one on its own to experiment with. Rutherford therefore decided to look at a thin metal sheet, gold wasn’t the ideal choice as it can be rolled out into extremely thin sheets of foil. This gold foil would have been a few atoms deep.
  • Gold was also a suitable choice for material as a single atom was much heavier than that of an alpha particle. This was important because if the material was a similar size (or mass) to that of an alpha particle, conservation of momentum tells us that either the alpha particles would bombard their way through, of the atoms of the foil itself would be ripped off – not ideal since the point was to determine the structure of the foil atoms themselves.

The Evacuated Chamber

  • The experiment had to be performed in an evacuated chamber because alpha particles are absorbed by a few centimetres of air. Therefore the air had to be removed so that the alpha particles could hit the detecting screen itself.

The Detecting Screen

  • The detecting instrument was made from Zinc Sulphide. Zinc Sulphide emits a photon of visible light when it is struck by a charged particle. Therefore with an alpha particle incident upon it, it was show up to the human eye by visible light being emitted at the point it is struck.
  • Zinc Sulphide was placed over the lens of a microscope so that a viewer could ‘see’ and therefore count the number of impacts.

Measuring the Angle

  • The arm of the microscope had to be placed on a turntable with a vernier scale etched onto it. Therefore as the angle the microscope viewed increased from a perpendicular position of the gold foil (on the opposite side to the radioactive isotope), call this  0 \deg , the number of detected alpha particles (scintillations) could be monitored.
  • As the angle increased, the number of detections decreased.
  • What was astonishing at the time was the number that were deflected back the way they came (or at least back to the same side of the gold foil as that of the radioactive isotope). Rutherford commented ‘It was quite the most incredible event that has happened to me in my life! It was almost as incredible as if you fired a 15″ shell at a piece of tissue paper and it came back and hit you!’

The ‘Backfired’ Alpha Particles

  • With the experiment carried out just once, the ‘backfired’ alpha particles would have been considered anomalies. The experiment would have needed to have been repeated multiple times for the ‘anomalies’ to have been questioned further.
  • It is the careful observations of these anomalies that helped to determine be structure of the atom. It would have been easy for the anomalies to have been ignored,  the results would then have helped to support the then ‘Plum Pudding Model’.

Approximately 1 in 8000 alpha particle were deflected back the way it came. This means that a lot of tedious experimental work would have needed to have taken place before any findings would be publicised. Nowadays data loggers could have been used which would provide much more reliable outcomes and quickly.

Further reading:

  • BBC Bitesize – link