Exploration of the Rutherford Experiment Simulation
The purpose of this exploration is to become familiar with the simulation of a simplified version
of Rutherford’s scattering experiment. As you work through this exercise, have the applet open
in a separate window (if necessary click on the link, historical_scattering2.swf). If you need help
understanding the apparatus and variables, see the Help menu for assistance before starting this
exploration.
Once you have finished this exploration, you should be able to conduct a simulation of
Rutherford’s experiment and complete a formal lab report (see Simulate Experiment and
Additional Experiments under the Resources menu of the applet).
Design of the Apparatus
Rutherford and his colleague, Hans Geiger, had been studying alpha particles and had developed
special zinc sulfide screens that showed a brief flash of light (called a scintillation) when an
alpha particle struck the screen. Ernest Marsden, at the time, was a graduate student in
Rutherford’s lab. Rutherford wrote:
… One day Geiger came to me and said, “Don’t you think that young Marsden, whom I am
training in radioactive methods, ought to begin a small research?” Now I had thought that, too,
so I said, “Why not let him see if any –particles can be scattered through a large angle?” I
may tell you in confidence that I did not believe that they would be, since we knew that the –
particle was a very fast, massive particle with a great deal of [kinetic] energy, and you could
show that if the scattering was due to the accumulated effect of a number of small scatterings,
the chance of an –particle’s being scattered backward was very small… (as quoted in Project
Physics, Unit 5, Models of the Atom, pp. 68-69, ©1975, Harvard Project Physics)
Thus, the almost circular zinc sulfide screen surrounding the metal foil (modeled in the applet)
was created and Marsden set to work under Geiger’s supervision. Geiger and Marsden spent a
great deal of time counting over one hundred thousand individual scintillations.
Scintillation Measurements
With the applet set to its default values (one filter, 0o, gold, 18.0 MeV alpha particles), click the
“Fire Particles” button. Click this button again to stop firing alpha particles when the red light
indicating saturation comes on.
1. Approximately how many (simulated) seconds and how many scintillations does it take
to reach saturation? You may need to click “Reset scintillator” and try this several times.
Geiger and Marsden did not have a digital counter (like the table in the applet) and had to rely on
a visual count of the scintillations viewed with a lens positioned at different angles around the
screen. The circular viewing area shown in the applet (top right corner) is a representation of
what they would have seen through their lens. After a certain number of scintillations (500 in this
simulation), the screen becomes saturated and visual counting is no longer possible. [Note that
the counter stops counting scintillations once saturation is reached, indicated by the red dot.]
Observations of scattered alpha particles can be made at different angles from the horizontal.
Geiger and Marsden would move their lens; you can simulate this by clicking and dragging the
grey box along the circular scintillation screen.
2. What is the maximum angle of the viewer that you can use in order to stop the count just
before the saturation limit of 500 is reached? Try to stop the counter reliably in the range
of 475 to 500 (without triggering the red saturation light).
Notice that, for small angles, it is very difficult to stop the counter just before saturation and do
this in a reproducible way. To solve the problem of saturation, Geiger and Marsden used filters at
the alpha-particle source to limit the number of alpha particles to a countable number.
3. Use the “Add filter” button to add sufficient filters so that you can reliably stop the
counter just before saturation at 0o. How many filters did you use?
4. Repeat #3 for angles between 0o and your answer to #2.
For the results to be statistically significant it is important to count the greatest number of
scattered alpha particles. Ideally this means as close as possible to 500 scintillations.
5. Try collecting results for several angles greater than 90o. Are any additional filters
needed? What other practical problem now arises?
Although you want to count as many scattered alpha particles as possible (before saturation), a
long time may be required at large angles. For the results to be statistically valid, always make
sure you run the simulation close to saturation (475-500) or for at least several hundred million
incident alpha particles.
6. Set the angle of the viewer to +145o. How many scintillations are counted and how many
seconds have elapsed if the simulation is stopped at approximately two hundred million
incident particles?
Scintillation Rate
For each of the following trials, use the appropriate number of filters to obtain suitable results.
7. Reset the scintillator and set the viewer at 0o. Run the simulation until just before
saturation and stop firing particles. Click “Record Data” button.
8. Calculate and record the percentage of particles reaching the scintillator.
9. Repeat #7 and #8 several times using different small (<90o) angles. Be sure to click the
“Record Data” button at the end of each trial.
10. In the Options menu, click Evidence to show the table of your results. How does the
scintillation rate compare with the percentages you calculated?
11. Write a word equation to define the scintillation rate.
To obtain a reliable scintillation rate, you need to run the simulation so that the number of
scintillations is as close as possible to the saturation point and conduct multiple repetitions using
the same set of variables (angle, filters, number of incident particles, element, and alpha particle
energy). For small angles this is easy to do; for large angles you may need to compromise by
using several hundred million incident particles and not reach the saturation point. In all cases,
an average of the scintillation rates for multiple repetitions will produce the most reliable result.
Significance of Rutherford’s Experiment
The legacy of Rutherford’s experiment includes two important advances in science. First and
foremost is the revision of the Thomson model of the atom and the introduction of the nuclear
model that remains today as a general concept. Secondly, the clever experimental design of a
scattering experiment (firing small charged particles at an unknown structure and studying the
scattering) is a design that has been used by many subsequent researchers in atomic physics.