TABLE OF CONTENTS The series of measurements performed by Hans Geiger and Ernest Marsden under Rutherford s direction at Manchester provide a classic example of a fixed target experiment. The target was a thin metal foil of relatively large atomic number, while the projectiles consisted of a collimated beam of low energy ?-particles, which, as we will see in the next chapter, are nothing more than the nuclei of helium atoms. The basic outcome of these experiments was that most of the ?-particles went straight through the foil with very little angular deviation. Occasionally, however, the deflections were quite large. A detailed analysis of these observations revealed the structure of the target, which ultimately led to the nuclear model of the atom.
To fully appreciate the beauty of these experiments, it is essential to analyze the results in their proper historical context. Prior to this work, the only popular model of the atom was due to Joseph Thomson, who visualized the electrically neutral atom as a plum pudding where negatively charged electrons were embedded, like raisins, within a uniform distribution of positive charge. If this model were correct, one would expect only small deviations in the ?-particles trajectories (primarily due to scattering from the electrons), unlike what was found by Geiger and Marsden. To see this, let us do a few simple kinematic calculations. Because the velocities of the ?-particles in these experiments were well below 0.1 c (where c refers to the speed of...
