Rutherford's alpha scattering experiment
Physics Narrative for 14-16
This is an historic experiment which is often presented to students with little context. It is suggested that, in 1910, the ‘plum pudding model’ was suddenly overturned by Rutherford’s experiment. In fact, Rutherford had already formulated the nuclear model of the atom before the experiment was carried out; his model allowed him to carry out a mathematical analysis of the data gathered by Geiger and Marsden.
Working with alpha radiation
Alpha and beta radiations were identified by Rutherford in 1899. Rutherford and Royds showed that an alpha particle was a helium-4 nucleus in 1909.
Rutherford knew that alpha radiation had a range of about 5 cm in air, and its range in denser materials had been measured.
Experiment: Alpha, beta and gamma radiations can be distinguished by their penetrating powers.
It was clear to Rutherford and many others that, since alpha and beta radiations came from atoms, they were on the right scale for use as probes of matter at the atomic scale. He and his colleagues developed practical sources of alpha radiation. When scientists are working at the frontiers, it is rare that they can buy their equipment and supplies ‘off the shelf’; each part of the experimental technique has to be devised, and equipment has to be designed and built.
Models of the atom
Rutherford and his team were working at Manchester University. At the Cavendish Laboratory, Cambridge, CTR Wilson was developing the cloud chamber, a device for showing the tracks of radiation. It is likely that Rutherford saw examples of such tracks, perhaps as early as 1908. Mostly, alpha particle tracks show hundreds of tiny deflections; occasionally, major deviations can be seen. Rutherford guessed that the many small deflections were caused by collisions with the very light electrons, while the (very rare) big deflections arose from collisions with something representing a more concentrated part of the atom.
For a discussion of this, see the guidance note:
See the experiment:
It will help if students are familiar with the behaviour of colliding objects of different masses.
Evidence of back-scattering
It is sometimes asked why, if alpha radiation was expected to pass right through the gold foil, the alpha-scattering experiment was designed to allow the detector to be moved round through an angle of greater than 90°. Of course, if the plum pudding model was correct, back-scattering would not be expected. However, as we saw above, Rutherford expected to observe back-scattering.
In fact, his colleagues had already observed the reflection of alpha particles by solid materials. This first came about when Geiger was investigating the penetration of mica (a transparent mineral) by alpha and beta radiation; he wanted to use thin sheets of mica as windows for sources of radiation, and in detectors.
Geiger and Marsden carried out an experiment in which alpha radiation was directed at different metals, and the reflected radiation detected. In their diagram:
AB is the source of radiation; it is a glass tube containing radioactive gas.
RR is the metal target, which reflects the radiation.
S is the scintillating screen which emits light when struck by radiation.
P is a barrier to prevent direct irradiation of the screen.
M is the viewing microscope through which the screen is observed.
Hence Rutherford knew that back-scattering was likely, and designed his experiment to observe it. His calculations allowed him to predict the pattern of the results, if not the actual fraction of the radiation which would be back-scattered at any particular angle.
There is a strong interplay between experimentation and theory in physics. Experimental results may help to alter theories, but theories also help to design experiments.
Rutherford is often quoted, describing his reaction to the alpha particle scattering experiment in words such as these:
"It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you."
Could Rutherford have been so surprised, when back-scattering had already been observed? He was an enthusiastic popularizer of his work; it is likely that, with these words, he was trying to show his audience how they might respond to his discovery.
We are grateful to David Sang, author of this Case Study.