The Rutherford-Geiger-Marsden Experiment

April 11, 2017 Alpha Spectroscopy,*English Posts*25,288 Views

What made by*Rutherford*and his assistants*Geiger*and*Marsden*is perhaps>The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

In the experiment, Rutherford sent a beam of*alpha particles*(helium nuclei) emitted from a radioactive source against a*thin gold foil*(the thickness of about 0.0004 mm, corresponding to about 1000 atoms).

Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as*pudding model) or if there was something different.

In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with*large angle of deviation. Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a*small and compact nucleus, but with high charge density, surrounded by an electron cloud.
In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil;>

The interaction between*an alpha particle and the nucleus (elastic collision) is also known as*Coulomb scattering, because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.

Experimental Setup

In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in*alpha spectroscopy*we built a setup based rel="noopener noreferrer" target="_blank">Alpha Spectrometer,*Gold Leaf Thickness*.
The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection.
The images below show the experimental setup:

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The alpha source is actually 0.9 μCi of*Am 241*(from smoke detector) which emits alpha particles with energy of 5.4 MeV. The alpha particle beam is collimated by a simple hole in a wooden screen. Source and collimator are fixed>

Results

With the experimental setup described above, we have carried out a series of measurements. We chose a measurement time of*10 min = 600 s. It is a rather short interval, especially for higher angles that give a few events, but sufficient to have a result of a qualitative nature. With a few events the statistical uncertainty is high, and then the corresponding data should be regarded>In addition has been placed a threshold of minimum energy of*700 KeV*so as to exclude from the count any spurious events that may occur, but that usually remain confined within the energy values of less than 1 MeV.
The results obtained are shown in the table. The rotating arm has been positioned at different angles, starting from a null angle up to a value of 75°, with steps of 15°. At angles greater than 45° events counted have been reduced to a few units. For these angles it would be necessary to increase the measurement time, or increase the power of the source so as to acquire a greater number of events and thus have a greater statistical significance.
The results of measurements are shown in the following graphs, in a linear scale and in a semi-logarithmic scale.

Linear Scale :

Semilog Scale

The results obtained in our experiment approach, albeit with obvious limitations, to the expected theoretical results, represented in the following graph:

For completeness, we report also at the side the formula that describes the distribution of the number of the counted particles in function of the scattering angle. Interestingly, this depends>


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