N.P. Armitage notes on the Rutherford Scattering Experiment
After discovery of the electron and the measurement of its charge and mass, attention turned to the microscopic structure of matter. In 1906 Thomson proposed that electrons were suspended in an background of otherwise uniform positive charge. This was his so-called "Plum Pudding Model" proposed in his seminal paper "On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure". Note that that is the title, not the abstract!
Rutherford, who recently had discovered that alpha particles could be
slightly deflected on passing through dense matter, thought that a systematic
study of alpha particle scattering may shed light on the density of Thomson's
pudding. A number of such studies were made. In 1909, an undergraduate
student working in Rutherford's lab, Ernest Marsden, was given the project of
studying alpha-particle scattering through a large angle. Rutherford
privately believed such a phenomena was probably not possible as they believed
the alpa was a very fast massive particle with so much energy that it was
unlikely that it could be deflected by much, but he thought it would be a good
training exercise for Marsden. To his surprise Geiger (of Geiger counter
fame) came back some time later to reprt with great excitement that "We have
been able to get some of the alpha-particles coming backward …" Rutherford
later wrote that "It was quite the most incredible event that ever happened to
me in my life. It was almost as incredible as if you fired a 15-inch shell at a
piece of tissue paper and it came back and hit you."
Plum pudding vs. Nuclear models
These observations were the fatal blow to the Plum Pudding model. The central assumption of that theory was that the positive charge was uniformly distributed over the atomic volume. For a massive Helium atom to be deflected through 180 degree was only possible if the positive charge was concentrated in small discrete pieces. Rutherford pondered his results for a number of months and finally decided that the only way they made sense was if the nuclear charge was concentrated in a volume much smaller than the atom size i.e. he postulated the nucleus! We now know that Rutherford was right, but this was a revolutionary suggestion at the time.
In this experiment you will attempt to confirm Rutherford, Geiger, and Marsden's experiment and investigate the angular distribution of scattered particles. Our experimental tool is essentially the same as Marsden's. We bombard a thin gold foil of gold with an alpha particle beam and count the rates scattered into various directions in space. We then compare our measured angular distribution to the Rutherford formula for scattering.
Information on the Rutherford scattering chamber and instructions on how to operate it can be found here.
Note that data acquisition in this experiment can take a long long time at angles greater than 30 degrees! It may be necessary to take data at a few of these angles for over 12-24 hours each! The apparatus is quite stable, so can be left running for some time. I advise putting a note on it however saying "DON'T TOUCH!". You will need to decide how many data points to take at these large angles. Be judicious to maximize your time, but it is expected that you should take enough to see proper backscattering and confirm the Rutherford scattering equation. This is the most time intensive lab that you will do in the course!
I also advise taking a little data with the aluminum target and following the recommendations in the manual. It is useful to compare with the gold target.
Useful Links:
Professor Petar Maksimovic's excellent writeup on these experiments
A nice historical overview of Rutherford and his experiments.
Leybold webpage on the Rutherford scattering chamber
